diff --git a/alembic/versions/0099_library_placement_run.py b/alembic/versions/0099_library_placement_run.py
new file mode 100644
index 0000000..8907a7f
--- /dev/null
+++ b/alembic/versions/0099_library_placement_run.py
@@ -0,0 +1,99 @@
+"""library_placement_run — the placement reconciler's plan/apply/undo ledger.
+
+Milestone #421 step 3. The survey (#4245) measured 33,789 ImageRecord rows
+sitting outside their artist's canonical directory, across 56 artists. This
+table holds one run of the sweep that trues them up: the plan, what it did,
+and where every file came from.
+
+## Why the moves live in a table rather than a log line
+
+`ImageRecord.path` is the only pointer at the bytes, so a move rewrites the
+row. Once that write lands, the previous location exists nowhere — unless it
+was recorded first. `moves` is that record, which is what makes a 33,789-file
+operation something the operator can undo per artist after looking at the
+result, rather than a one-way door.
+
+An `applied` row is therefore HISTORY, not state (lesson #4226). Any future
+retention on this table may prune `ready`, `cancelled` and `error` runs; an
+`applied` one is only disposable once someone decides undo is no longer
+wanted. That is deliberately not a timer's decision, and no pruning is added
+here.
+
+Revision ID: 0099
+Revises: 0098
+Create Date: 2026-09-21
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision: str = "0099"
+down_revision: Union[str, None] = "0098"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "library_placement_run",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column(
+            "status", sa.String(length=16), server_default="running",
+            nullable=False,
+        ),
+        # SET NULL, not CASCADE: deleting an artist must not destroy the
+        # record of where their files were moved.
+        sa.Column("artist_id", sa.Integer(), nullable=True),
+        sa.Column(
+            "started_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=False,
+        ),
+        sa.Column("finished_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column(
+            "planned_count", sa.Integer(), server_default="0", nullable=False,
+        ),
+        sa.Column(
+            "moved_count", sa.Integer(), server_default="0", nullable=False,
+        ),
+        sa.Column(
+            "refused_count", sa.Integer(), server_default="0", nullable=False,
+        ),
+        sa.Column(
+            "moves", postgresql.JSONB(astext_type=sa.Text()),
+            server_default=sa.text("'[]'::jsonb"), nullable=False,
+        ),
+        sa.Column(
+            "refusals", postgresql.JSONB(astext_type=sa.Text()),
+            server_default=sa.text("'[]'::jsonb"), nullable=False,
+        ),
+        sa.Column("error", sa.Text(), nullable=True),
+        sa.ForeignKeyConstraint(
+            ["artist_id"], ["artist.id"],
+            name="fk_library_placement_run_artist_id", ondelete="SET NULL",
+        ),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index(
+        "ix_library_placement_run_status", "library_placement_run", ["status"],
+    )
+    op.create_index(
+        "ix_library_placement_run_artist_id", "library_placement_run",
+        ["artist_id"],
+    )
+
+
+def downgrade() -> None:
+    # Dropping this table destroys the only record of where moved files came
+    # from. That is correct for a downgrade — the code that reads it is going
+    # away too — but it is worth saying out loud rather than discovering.
+    op.drop_index(
+        "ix_library_placement_run_artist_id",
+        table_name="library_placement_run",
+    )
+    op.drop_index(
+        "ix_library_placement_run_status", table_name="library_placement_run",
+    )
+    op.drop_table("library_placement_run")
diff --git a/backend/app/api/cleanup.py b/backend/app/api/cleanup.py
index 44cf6d3..d4b2196 100644
--- a/backend/app/api/cleanup.py
+++ b/backend/app/api/cleanup.py
@@ -29,7 +29,7 @@ from quart import Blueprint, jsonify, request
 from sqlalchemy import select
 
 from ..extensions import get_session
-from ..models import LibraryAuditRun
+from ..models import LibraryAuditRun, LibraryPlacementRun
 from ..services import cleanup_service, library_layout
 from ._responses import error_response as _bad
 
@@ -218,3 +218,91 @@ async def layout_survey():
             )
         )
     return jsonify({**report.as_dict(), "checked_disk": check_disk})
+
+
+def _serialize_placement_run(run: LibraryPlacementRun, *, moves: bool = False) -> dict:
+    """`moves` is opt-in: an applied whole-library run carries tens of
+    thousands of entries, which is a fine thing to hold in Postgres and a
+    poor thing to put in every list response."""
+    out = {
+        "id": run.id,
+        "status": run.status,
+        "artist_id": run.artist_id,
+        "started_at": run.started_at.isoformat() if run.started_at else None,
+        "finished_at": run.finished_at.isoformat() if run.finished_at else None,
+        "planned_count": run.planned_count,
+        "moved_count": run.moved_count,
+        "refused_count": run.refused_count,
+        "refusals": run.refusals or [],
+        "error": run.error,
+    }
+    if moves:
+        out["moves"] = run.moves or []
+    return out
+
+
+@cleanup_bp.route("/placement/runs", methods=["GET"])
+async def placement_runs():
+    """Newest first. Without `moves`, so the list stays small."""
+    try:
+        limit = min(int(request.args.get("limit", "25")), 100)
+    except ValueError:
+        return _bad("invalid_limit")
+    async with get_session() as session:
+        rows = (await session.execute(
+            select(LibraryPlacementRun)
+            .order_by(LibraryPlacementRun.id.desc()).limit(limit)
+        )).scalars().all()
+    return jsonify({"runs": [_serialize_placement_run(r) for r in rows]})
+
+
+@cleanup_bp.route("/placement/runs/<int:run_id>", methods=["GET"])
+async def placement_run(run_id: int):
+    """One run WITH its moves — this is the preview the operator reads before
+    agreeing, and the record of what happened afterwards."""
+    async with get_session() as session:
+        run = await session.get(LibraryPlacementRun, run_id)
+        if run is None:
+            return _bad("not_found", status=404)
+        return jsonify(_serialize_placement_run(run, moves=True))
+
+
+@cleanup_bp.route("/placement/plan", methods=["POST"])
+async def placement_plan():
+    """Queue a planning run. `artist_id` scopes it to one artist, which is the
+    intended use: do one, look at the gallery, then continue or revert."""
+    body = await request.get_json(silent=True) or {}
+    artist_id = body.get("artist_id")
+    if artist_id is not None and not isinstance(artist_id, int):
+        return _bad("invalid_artist_id")
+    from ..tasks.library_placement import plan_placement
+    plan_placement.delay(artist_id)
+    return jsonify({"status": "dispatched"}), 202
+
+
+@cleanup_bp.route("/placement/runs/<int:run_id>/apply", methods=["POST"])
+async def placement_apply(run_id: int):
+    """Execute a ready run. This renames files and rewrites rows."""
+    async with get_session() as session:
+        run = await session.get(LibraryPlacementRun, run_id)
+        if run is None:
+            return _bad("not_found", status=404)
+        if run.status != "ready":
+            return _bad("not_ready", detail=f"run is {run.status}")
+    from ..tasks.library_placement import apply_placement
+    apply_placement.delay(run_id)
+    return jsonify({"status": "dispatched"}), 202
+
+
+@cleanup_bp.route("/placement/runs/<int:run_id>/revert", methods=["POST"])
+async def placement_revert(run_id: int):
+    """Put an applied run's files back. The reason the ledger is kept."""
+    async with get_session() as session:
+        run = await session.get(LibraryPlacementRun, run_id)
+        if run is None:
+            return _bad("not_found", status=404)
+        if run.status != "applied":
+            return _bad("not_applied", detail=f"run is {run.status}")
+    from ..tasks.library_placement import revert_placement
+    revert_placement.delay(run_id)
+    return jsonify({"status": "dispatched"}), 202
diff --git a/backend/app/celery_app.py b/backend/app/celery_app.py
index d49c8ef..5f176a2 100644
--- a/backend/app/celery_app.py
+++ b/backend/app/celery_app.py
@@ -35,6 +35,7 @@ def make_celery() -> Celery:
             "backend.app.tasks.backup",
             "backend.app.tasks.admin",
             "backend.app.tasks.library_audit",
+            "backend.app.tasks.library_placement",
             "backend.app.tasks.translation",
         ],
     )
@@ -62,6 +63,8 @@ def make_celery() -> Celery:
             # 2026-06-07: a 2h audit blocked vacuum/backup/normalize for hours).
             "backend.app.tasks.maintenance.*": {"queue": "maintenance"},
             "backend.app.tasks.backup.*": {"queue": "maintenance_long"},
+            # 33k renames on NFS: long lane, same as backups.
+            "backend.app.tasks.library_placement.*": {"queue": "maintenance_long"},
             "backend.app.tasks.admin.*": {"queue": "maintenance_long"},
             "backend.app.tasks.library_audit.*": {"queue": "maintenance_long"},
             # Translation backfill hits the LLM (~1–6s/item) → the long lane so it
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index 9e085f4..b9bcf4a 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -22,6 +22,7 @@ from .import_batch import ImportBatch
 from .import_settings import ImportSettings
 from .import_task import ImportTask
 from .library_audit_run import LibraryAuditRun
+from .library_placement_run import LibraryPlacementRun
 from .membership_sync import MembershipSync
 from .ml_settings import MLSettings
 from .patreon_failed_media import PatreonFailedMedia
@@ -85,6 +86,7 @@ __all__ = [
     "ImportTask",
     "ImportSettings",
     "LibraryAuditRun",
+    "LibraryPlacementRun",
     "MembershipSync",
     "MLSettings",
     "HeadAutoApplyRun",
diff --git a/backend/app/models/library_placement_run.py b/backend/app/models/library_placement_run.py
new file mode 100644
index 0000000..da1fbfc
--- /dev/null
+++ b/backend/app/models/library_placement_run.py
@@ -0,0 +1,99 @@
+"""LibraryPlacementRun — one run of the placement reconciler (milestone #421).
+
+The library is keyed on the Artist row's `slug`, one directory per artist.
+Every writer agrees on that now (`utils.paths.canonical_subdir`, task #4244),
+but ~33,789 rows were written under older rules and sit in some other
+artist's directory. This row is a run of the sweep that trues them up.
+
+State machine, mirroring LibraryAuditRun:
+
+    running -> ready -> applied -> reverted
+                     \\-> cancelled
+    (any)   -> error
+
+## The `moves` column does three jobs
+
+`moves` is the plan: `[{"image_id": 1, "from": "...", "to": "..."}, ...]`.
+
+  1. **Preview.** It is what the operator reads before agreeing.
+  2. **Apply.** The apply executes THIS list rather than re-deriving the set,
+     so the preview cannot describe a different set from the apply. That is
+     rule 93's guarantee reached the way LibraryAuditRun reaches it — the
+     plan is materialised, not recomputed.
+  3. **Revert.** `from` is retained, so a batch that looks wrong in the
+     gallery goes back where it came from.
+
+## An applied run IS the undo ledger — it must never be pruned
+
+This is the trap lesson #4226 names: a record that answers both "what is the
+current plan" and "what happened" gets deleted by whatever forgets the first.
+A `ready` run is disposable state. An `applied` run is HISTORY, and it is the
+only record of where 33,789 files used to be — delete it and the moves become
+irreversible.
+
+No pruning exists for this table today, and that is deliberate. If retention
+is ever added here, it may prune `ready`, `cancelled` and `error` runs; an
+`applied` run is only safe to drop once someone decides the moves are settled
+and undo is no longer wanted, which is an operator decision and not a
+timer's.
+"""
+
+from datetime import datetime
+from typing import Any
+
+from sqlalchemy import DateTime, ForeignKey, Integer, String, Text, func, text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class LibraryPlacementRun(Base):
+    __tablename__ = "library_placement_run"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    status: Mapped[str] = mapped_column(
+        String(16), nullable=False, default="running", index=True,
+        server_default="running",
+    )
+    # running | ready | applied | reverted | cancelled | error
+
+    # Scope. NULL = the whole library; set = one artist, which is how this is
+    # meant to be used — do one artist, look at it in the gallery, continue or
+    # revert. ondelete SET NULL rather than CASCADE: deleting an artist must
+    # not destroy the record of where their files were moved.
+    artist_id: Mapped[int | None] = mapped_column(
+        ForeignKey("artist.id", ondelete="SET NULL"), nullable=True, index=True,
+    )
+
+    started_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(),
+    )
+    finished_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True,
+    )
+
+    planned_count: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0, server_default="0",
+    )
+    moved_count: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0, server_default="0",
+    )
+    refused_count: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0, server_default="0",
+    )
+
+    # [{"image_id": int, "from": str, "to": str}, ...] — see the module
+    # docstring. This is the plan, the audit trail and the undo, in that order
+    # of appearance and in one place.
+    moves: Mapped[list[dict[str, Any]]] = mapped_column(
+        JSONB, nullable=False, default=list, server_default=text("'[]'::jsonb"),
+    )
+    # [{"image_id": int, "reason": str}, ...] — rows the apply declined to
+    # touch, with why. A refusal is an expected outcome, not an error: the
+    # world moves between plan and apply, and every gate fails closed.
+    refusals: Mapped[list[dict[str, Any]]] = mapped_column(
+        JSONB, nullable=False, default=list, server_default=text("'[]'::jsonb"),
+    )
+
+    error: Mapped[str | None] = mapped_column(Text, nullable=True)
diff --git a/backend/app/services/library_layout.py b/backend/app/services/library_layout.py
index f1c2eeb..c14c711 100644
--- a/backend/app/services/library_layout.py
+++ b/backend/app/services/library_layout.py
@@ -16,17 +16,23 @@ writing their own — the house shape for rule 93, snippet #3087. A preview
 that computes its set differently from the apply is a preview that can lie,
 and here the apply RENAMES the operator's art.
 
-Nothing in this module writes. It reads rows, and it stats files when asked.
+## What writes and what does not
+
+`survey_layout` and `plan_placement` are read-only — they report and they
+record a plan. `apply_run` and `revert_run` are the only functions here that
+rename a file or rewrite a row, and each does both for one row at a time,
+updating the row only after its rename lands.
 """
 from __future__ import annotations
 
 from dataclasses import dataclass, field
+from datetime import UTC, datetime
 from pathlib import Path
 
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
-from ..models import Artist, ImageRecord
+from ..models import Artist, ImageRecord, LibraryPlacementRun
 from ..utils.paths import canonical_subdir
 
 # Top-level directories under the images root that are STORES, not artists.
@@ -229,3 +235,200 @@ def survey_layout(
         report.unmovable += layout.unmovable
 
     return report
+
+
+# --- the reconciler: plan -> apply -> revert (#4246) -------------------------
+#
+# The three verbs share one materialised plan rather than each deriving its
+# own set. `plan_placement` writes `LibraryPlacementRun.moves`; `apply_run`
+# executes THAT list; `revert_run` walks it backwards. A preview that can
+# disagree with its apply is the failure this shape exists to prevent, and
+# here the apply renames the operator's art.
+#
+# Every step fails CLOSED. The world moves between planning and applying —
+# a download lands, a supersede rewrites a path, a file is deleted — so the
+# apply re-checks each row against what the plan recorded and declines the
+# ones that moved on, instead of trusting a plan that may be minutes old.
+
+
+def plan_placement(
+    session: Session, images_root: Path, *, artist_id: int | None = None,
+) -> LibraryPlacementRun:
+    """Build (and persist) the move plan. Touches no files.
+
+    `artist_id` scopes the run to one artist, which is how this is meant to be
+    used: do one, look at the gallery, then continue or revert. None plans the
+    whole library.
+    """
+    stmt = select(Artist).order_by(Artist.slug)
+    if artist_id is not None:
+        stmt = stmt.where(Artist.id == artist_id)
+    artists = session.execute(stmt).scalars().all()
+
+    candidates: list[dict] = []
+    wanted: dict[str, int] = {}
+    for artist in artists:
+        rows = session.execute(
+            select(ImageRecord.id, ImageRecord.path)
+            .where(*_misplaced_conditions(images_root, artist.id, artist.slug))
+        ).all()
+        for row_id, path in rows:
+            dest = destination_for(path, images_root, artist.slug)
+            if dest is None or dest.exists():
+                continue
+            key = str(dest)
+            candidates.append({"image_id": row_id, "from": path, "to": key})
+            wanted[key] = wanted.get(key, 0) + 1
+
+    # Two rows wanting one destination: plan NEITHER. Which of them "wins" is
+    # not this sweep's call, and planning one of them would silently pick a
+    # winner by iteration order. Counting first and filtering after is what
+    # makes that true — claiming as we go would quietly keep whichever came
+    # first.
+    moves = [m for m in candidates if wanted[m["to"]] == 1]
+
+    run = LibraryPlacementRun(
+        status="ready", artist_id=artist_id, moves=moves,
+        planned_count=len(moves),
+    )
+    session.add(run)
+    session.flush()
+    return run
+
+
+def _move_one(src: Path, dest: Path) -> str | None:
+    """Rename `src` to `dest`. Returns a refusal reason, or None on success.
+
+    A rename within one filesystem, so no copy and no free space needed. The
+    destination check is not a race-free guarantee — nothing here is — but it
+    turns the common case of "something already landed there" into a refusal
+    instead of a silent overwrite.
+    """
+    if not src.exists():
+        return "source missing"
+    if dest.exists():
+        return "destination occupied"
+    try:
+        dest.parent.mkdir(parents=True, exist_ok=True)
+        src.rename(dest)
+    except OSError as exc:
+        return f"rename failed: {exc}"
+    return None
+
+
+def apply_run(
+    session: Session, run: LibraryPlacementRun, *, chunk: int = 0,
+) -> LibraryPlacementRun:
+    """Execute a `ready` run's stored plan: file and row together, per row.
+
+    The row is updated ONLY after its rename lands, so a refused or failed
+    move can never leave `ImageRecord.path` pointing at a file that is not
+    there. Refusals are recorded and the run continues — one row that moved
+    on since planning is not a reason to abandon the other 33,788.
+
+    `chunk` commits progress every N moves. Set it for any real run: the
+    ledger is the ONLY record of where a file came from, so a worker that
+    dies at row 30,000 of 33,789 must not take the undo information for the
+    first 29,999 with it. Left at 0 (tests, small runs) everything persists
+    in one go at the end.
+
+    Re-running a partially-applied plan is safe rather than clever: the rows
+    already moved no longer match their `from`, so they refuse as "row moved
+    since planning" instead of being moved twice.
+    """
+    if run.status != "ready":
+        raise ValueError(f"run {run.id} is {run.status}, not ready")
+
+    refusals: list[dict] = []
+    moved: list[dict] = []
+
+    def _persist() -> None:
+        # Reassign rather than mutate: SQLAlchemy does not track in-place
+        # changes to a JSONB list, so an .append() alone would never reach
+        # the database and the ledger would silently stay empty.
+        run.moves = list(moved)
+        run.refusals = list(refusals)
+        run.moved_count = len(moved)
+        run.refused_count = len(refusals)
+        session.commit()
+
+    # Snapshot the plan before iterating: `_persist` reassigns `run.moves`,
+    # and iterating the attribute while rewriting it would walk a list that
+    # changes underneath the loop.
+    plan = list(run.moves)
+    for done, move in enumerate(plan, start=1):
+        record = session.get(ImageRecord, move["image_id"])
+        if record is None:
+            refusals.append({"image_id": move["image_id"], "reason": "row gone"})
+            continue
+        if record.path != move["from"]:
+            # Something rewrote this row since the plan was built — a
+            # supersede, or an earlier run. The plan is stale for it.
+            refusals.append({
+                "image_id": move["image_id"], "reason": "row moved since planning",
+            })
+            continue
+        reason = _move_one(Path(move["from"]), Path(move["to"]))
+        if reason is not None:
+            refusals.append({"image_id": move["image_id"], "reason": reason})
+            continue
+        record.path = move["to"]
+        moved.append(move)
+        if chunk and done % chunk == 0:
+            _persist()
+
+    run.moves = moved
+    run.refusals = refusals
+    run.moved_count = len(moved)
+    run.refused_count = len(refusals)
+    run.status = "applied"
+    run.finished_at = datetime.now(UTC)
+    session.flush()
+    return run
+
+
+def revert_run(
+    session: Session, run: LibraryPlacementRun,
+) -> LibraryPlacementRun:
+    """Put an applied run's files back where they came from.
+
+    This is why `from` is retained. It is the answer to "do one artist, look
+    at it, and undo if it reads wrong" — which is a cheaper way to settle
+    whether artist_id or the folder held the truth (#4257) than arguing it
+    from a sample.
+
+    Refuses the same way the apply does: a file someone has since moved or
+    replaced stays where it is, and its row is left alone.
+
+    A revert interrupted half way is resumable by re-running it: the rows
+    already put back no longer sit at `to`, so they refuse rather than move
+    twice. Unlike the apply this needs no chunked persistence — it consumes
+    the ledger rather than producing it, so a crash costs progress, not
+    information.
+    """
+    if run.status != "applied":
+        raise ValueError(f"run {run.id} is {run.status}, not applied")
+
+    refusals: list[dict] = []
+    reverted = 0
+    for move in run.moves:
+        record = session.get(ImageRecord, move["image_id"])
+        if record is None or record.path != move["to"]:
+            refusals.append({
+                "image_id": move["image_id"], "reason": "row changed since apply",
+            })
+            continue
+        reason = _move_one(Path(move["to"]), Path(move["from"]))
+        if reason is not None:
+            refusals.append({"image_id": move["image_id"], "reason": reason})
+            continue
+        record.path = move["from"]
+        reverted += 1
+
+    run.refusals = refusals
+    run.refused_count = len(refusals)
+    run.moved_count = run.moved_count - reverted
+    run.status = "reverted"
+    run.finished_at = datetime.now(UTC)
+    session.flush()
+    return run
diff --git a/backend/app/tasks/library_placement.py b/backend/app/tasks/library_placement.py
new file mode 100644
index 0000000..510795d
--- /dev/null
+++ b/backend/app/tasks/library_placement.py
@@ -0,0 +1,125 @@
+"""Placement reconciler tasks — plan, apply, revert (milestone #421).
+
+The service (`services.library_layout`) holds the decisions; this module is
+only the async wrapper, matching `tasks.library_audit`: run on the
+maintenance queue, mark the run `error` with a traceback if anything escapes,
+and return a small summary dict so eager-mode tests can assert on it.
+
+Applying is a long run — 33,789 renames on the operator's library at the time
+of writing — so `apply_placement` persists its ledger in chunks rather than
+at the end. That ledger is the only record of where each file came from, and
+a worker that dies two thirds of the way through must not take the undo
+information for the first two thirds with it.
+"""
+
+import logging
+import traceback
+from datetime import UTC, datetime
+from pathlib import Path
+
+from sqlalchemy.exc import DBAPIError, OperationalError
+
+from ..celery_app import celery
+from ..models import LibraryPlacementRun
+from ..services import library_layout
+from ._sync_engine import sync_session_factory as _sync_session_factory
+
+log = logging.getLogger(__name__)
+
+IMAGES_ROOT = Path("/images")
+
+# Commit the ledger every this many moves. Small enough that a crash loses
+# seconds of work, large enough not to make a COMMIT per rename.
+_APPLY_CHUNK = 200
+
+
+def _fail(session, run_id: int, message: str) -> None:
+    run = session.get(LibraryPlacementRun, run_id)
+    if run is not None:
+        run.status = "error"
+        run.error = message
+        run.finished_at = datetime.now(UTC)
+        session.commit()
+
+
+@celery.task(
+    name="backend.app.tasks.library_placement.plan_placement",
+    autoretry_for=(OperationalError, DBAPIError),
+    retry_backoff=5, retry_backoff_max=60, retry_jitter=True, max_retries=3,
+    soft_time_limit=900, time_limit=1000,
+)
+def plan_placement(artist_id: int | None = None) -> dict:
+    """Build a move plan and leave it `ready` for the operator to read.
+
+    Reads rows and stats destinations; moves nothing.
+    """
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        run = library_layout.plan_placement(
+            session, IMAGES_ROOT, artist_id=artist_id,
+        )
+        session.commit()
+        return {
+            "run_id": run.id, "status": run.status,
+            "planned_count": run.planned_count,
+        }
+
+
+@celery.task(
+    name="backend.app.tasks.library_placement.apply_placement",
+    soft_time_limit=7200, time_limit=7500,
+)
+def apply_placement(run_id: int) -> dict:
+    """Execute a `ready` run's stored plan. Renames files and rewrites rows.
+
+    No autoretry: a retry would re-enter a half-applied plan on a schedule
+    nobody asked for. Re-running IS safe (the applied rows refuse as "row
+    moved since planning"), but that should be the operator's decision after
+    reading what happened, not the queue's.
+    """
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        run = session.get(LibraryPlacementRun, run_id)
+        if run is None:
+            return {"run_id": run_id, "status": "missing"}
+        if run.status != "ready":
+            return {"run_id": run_id, "status": run.status, "skipped": True}
+        try:
+            library_layout.apply_run(session, run, chunk=_APPLY_CHUNK)
+            session.commit()
+        except Exception:
+            log.exception("placement apply failed for run %s", run_id)
+            session.rollback()
+            _fail(session, run_id, traceback.format_exc())
+            return {"run_id": run_id, "status": "error"}
+        return {
+            "run_id": run_id, "status": run.status,
+            "moved": run.moved_count, "refused": run.refused_count,
+        }
+
+
+@celery.task(
+    name="backend.app.tasks.library_placement.revert_placement",
+    soft_time_limit=7200, time_limit=7500,
+)
+def revert_placement(run_id: int) -> dict:
+    """Put an applied run's files back where they came from."""
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        run = session.get(LibraryPlacementRun, run_id)
+        if run is None:
+            return {"run_id": run_id, "status": "missing"}
+        if run.status != "applied":
+            return {"run_id": run_id, "status": run.status, "skipped": True}
+        try:
+            library_layout.revert_run(session, run)
+            session.commit()
+        except Exception:
+            log.exception("placement revert failed for run %s", run_id)
+            session.rollback()
+            _fail(session, run_id, traceback.format_exc())
+            return {"run_id": run_id, "status": "error"}
+        return {
+            "run_id": run_id, "status": run.status,
+            "refused": run.refused_count,
+        }
diff --git a/frontend/src/components/settings/MaintenancePanel.vue b/frontend/src/components/settings/MaintenancePanel.vue
index 3931384..0c44eaa 100644
--- a/frontend/src/components/settings/MaintenancePanel.vue
+++ b/frontend/src/components/settings/MaintenancePanel.vue
@@ -54,6 +54,7 @@
         Self-healing and repair: missing files, thumbnails, database upkeep.
       </p>
       <div class="fc-tile-grid">
+        <PlacementCard />
         <MissingFileRepairCard />
         <ThumbnailBackfillCard />
         <DbMaintenanceCard />
@@ -80,6 +81,7 @@ import MLBackfillCard from './MLBackfillCard.vue'
 import ThumbnailBackfillCard from './ThumbnailBackfillCard.vue'
 import ArchiveReextractCard from './ArchiveReextractCard.vue'
 import MissingFileRepairCard from './MissingFileRepairCard.vue'
+import PlacementCard from './PlacementCard.vue'
 import GpuTriageCard from './GpuTriageCard.vue'
 import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import VideoEmbeddingCard from './VideoEmbeddingCard.vue'
diff --git a/frontend/src/components/settings/PlacementCard.vue b/frontend/src/components/settings/PlacementCard.vue
new file mode 100644
index 0000000..462c255
--- /dev/null
+++ b/frontend/src/components/settings/PlacementCard.vue
@@ -0,0 +1,312 @@
+<template>
+  <MaintenanceTile
+    icon="mdi-folder-move"
+    title="File placement"
+    blurb="Put every image in its own artist's folder, one artist at a time."
+  >
+    <p class="fc-muted text-body-2 mb-4">
+      The library keeps one folder per artist, named after them. Files written
+      under older rules can sit in another artist's folder — this moves them
+      home, updating the record and the file together. Every run can be
+      reverted, so the safe way to use it is one artist at a time: run it,
+      look at the gallery, then continue or put it back.
+    </p>
+
+    <v-alert
+      v-if="error" type="warning" variant="tonal" density="compact" class="mb-3"
+    >{{ error }}</v-alert>
+
+    <!-- Survey -->
+    <div class="fc-settings-row mb-3">
+      <v-btn
+        variant="tonal" rounded="pill" prepend-icon="mdi-magnify"
+        :loading="surveying" @click="onSurvey"
+      >Check placement</v-btn>
+      <span v-if="layout" class="fc-muted text-body-2">
+        {{ layout.misplaced_rows.toLocaleString() }} of
+        {{ layout.total_rows.toLocaleString() }} images are in the wrong folder
+        <template v-if="layout.misplaced_rows">
+          — across {{ layout.artists.length }} artists
+        </template>
+      </span>
+    </div>
+
+    <v-table v-if="layout && layout.artists.length" density="compact" class="mb-2">
+      <thead>
+        <tr>
+          <th>Artist</th>
+          <th class="text-right">To move</th>
+          <th>Currently in</th>
+          <th class="text-right">Plan</th>
+        </tr>
+      </thead>
+      <tbody>
+        <tr v-for="a in layout.artists" :key="a.artist_id">
+          <td>{{ a.name }}</td>
+          <td class="text-right fc-tabular">{{ a.misplaced_rows.toLocaleString() }}</td>
+          <td class="fc-muted text-body-2">{{ a.stray_dirs.join(', ') }}</td>
+          <td class="text-right">
+            <v-btn
+              size="small" variant="text" :loading="planningId === a.artist_id"
+              @click="onPlan(a.artist_id)"
+            >Plan</v-btn>
+          </td>
+        </tr>
+      </tbody>
+    </v-table>
+
+    <v-divider class="my-4" />
+
+    <!-- Runs -->
+    <h3 class="fc-section-title">Runs</h3>
+    <v-table density="compact">
+      <thead>
+        <tr>
+          <th>When</th>
+          <th>Scope</th>
+          <th>Status</th>
+          <th class="text-right">Planned</th>
+          <th class="text-right">Moved</th>
+          <th class="text-right">Refused</th>
+          <th class="text-right">Actions</th>
+        </tr>
+      </thead>
+      <tbody>
+        <tr v-for="r in store.placementRuns" :key="r.id">
+          <td class="fc-tabular" :title="r.started_at">
+            {{ formatRelative(r.started_at) }}
+          </td>
+          <td>{{ artistName(r.artist_id) }}</td>
+          <td>
+            <v-icon size="small" :color="statusColor(r.status)">
+              {{ statusIcon(r.status) }}
+            </v-icon>
+            {{ r.status }}
+          </td>
+          <td class="text-right fc-tabular">{{ r.planned_count.toLocaleString() }}</td>
+          <td class="text-right fc-tabular">{{ r.moved_count.toLocaleString() }}</td>
+          <td class="text-right fc-tabular">
+            <span :class="r.refused_count ? 'text-warning' : 'fc-muted'">
+              {{ r.refused_count.toLocaleString() }}
+            </span>
+          </td>
+          <td class="text-right">
+            <KebabMenu :label="`Actions for placement run ${r.id}`">
+              <v-list-item title="Review…" @click="onReview(r)" />
+              <v-list-item
+                title="Move files" :disabled="r.status !== 'ready' || !r.planned_count"
+                @click="onApply(r)"
+              />
+              <v-list-item
+                title="Put back…" :disabled="r.status !== 'applied'"
+                @click="onRevert(r)"
+              />
+            </KebabMenu>
+          </td>
+        </tr>
+        <tr v-if="!store.placementRuns.length">
+          <td colspan="7" class="text-center fc-muted py-4">
+            No runs yet. Check placement above, then plan one artist.
+          </td>
+        </tr>
+      </tbody>
+    </v-table>
+
+    <!-- Review: the plan, which is what gets agreed to -->
+    <v-dialog v-model="reviewOpen" max-width="900">
+      <v-card>
+        <v-card-title>
+          Run {{ reviewRun?.id }} — {{ reviewRun?.planned_count?.toLocaleString() }} moves
+        </v-card-title>
+        <v-card-text>
+          <p class="fc-muted text-body-2 mb-3">
+            Showing the first {{ REVIEW_LIMIT }}. Each row moves the file and
+            its record together; nothing is overwritten.
+          </p>
+          <v-table density="compact">
+            <tbody>
+              <tr v-for="m in reviewMoves" :key="m.image_id">
+                <td class="fc-muted text-caption">{{ m.from }}</td>
+                <td class="text-caption">→ {{ m.to }}</td>
+              </tr>
+            </tbody>
+          </v-table>
+          <div v-if="reviewRun?.refusals?.length" class="mt-4">
+            <h4 class="fc-section-title">Refused ({{ reviewRun.refusals.length }})</h4>
+            <p class="fc-muted text-body-2">
+              Rows the run declined to touch — the source moved, the
+              destination was taken, or the record changed since planning.
+            </p>
+            <div
+              v-for="f in reviewRun.refusals.slice(0, REVIEW_LIMIT)" :key="f.image_id"
+              class="text-caption fc-muted"
+            >#{{ f.image_id }} — {{ f.reason }}</div>
+          </div>
+        </v-card-text>
+        <v-card-actions>
+          <v-spacer />
+          <v-btn variant="text" @click="reviewOpen = false">Close</v-btn>
+        </v-card-actions>
+      </v-card>
+    </v-dialog>
+
+    <v-dialog v-model="confirmOpen" max-width="520">
+      <v-card>
+        <v-card-title>{{ confirmTitle }}</v-card-title>
+        <v-card-text class="text-body-2">{{ confirmMessage }}</v-card-text>
+        <v-card-actions>
+          <v-spacer />
+          <v-btn variant="text" @click="confirmOpen = false">Cancel</v-btn>
+          <v-btn
+            color="accent" variant="flat" rounded="pill"
+            @click="confirmOpen = false; onConfirmed()"
+          >Go ahead</v-btn>
+        </v-card-actions>
+      </v-card>
+    </v-dialog>
+  </MaintenanceTile>
+</template>
+
+<script setup>
+import { computed, onMounted, onUnmounted, ref } from 'vue'
+
+import KebabMenu from '../common/KebabMenu.vue'
+import MaintenanceTile from '../common/MaintenanceTile.vue'
+import { useCleanupStore } from '../../stores/cleanup.js'
+import { formatRelative } from '../../utils/date.js'
+import { toast } from '../../utils/toast.js'
+
+// A whole-library plan is tens of thousands of moves. The dialog is for
+// judging whether the moves look right, which a sample answers — the full
+// list is in the run row if anyone needs it.
+const REVIEW_LIMIT = 200
+
+const store = useCleanupStore()
+const layout = ref(null)
+const artists = ref({})
+const surveying = ref(false)
+const planningId = ref(null)
+const error = ref('')
+
+const reviewOpen = ref(false)
+const reviewRun = ref(null)
+const reviewMoves = computed(() => (reviewRun.value?.moves || []).slice(0, REVIEW_LIMIT))
+
+const confirmOpen = ref(false)
+const confirmTitle = ref('')
+const confirmMessage = ref('')
+let confirmAction = null
+
+let poll = null
+
+function artistName(id) {
+  if (id === null || id === undefined) return 'Whole library'
+  return artists.value[id] || `#${id}`
+}
+
+function statusIcon(s) {
+  return {
+    ready: 'mdi-clipboard-text-outline', applied: 'mdi-check-circle',
+    reverted: 'mdi-undo-variant', error: 'mdi-close-circle',
+    running: 'mdi-timer-sand', cancelled: 'mdi-cancel',
+  }[s] || 'mdi-help-circle'
+}
+function statusColor(s) {
+  return {
+    ready: 'accent', applied: 'success', reverted: 'info',
+    error: 'error', running: 'warning',
+  }[s] || 'grey'
+}
+
+async function guard(fn) {
+  error.value = ''
+  try {
+    return await fn()
+  } catch (e) {
+    error.value = e?.message || String(e)
+  }
+}
+
+async function onSurvey() {
+  surveying.value = true
+  try {
+    await guard(async () => { layout.value = await store.loadLayout() })
+  } finally {
+    surveying.value = false
+  }
+}
+
+async function onPlan(artistId) {
+  planningId.value = artistId
+  try {
+    await guard(async () => {
+      await store.planPlacement(artistId)
+      toast('Planning — the run appears below when it is ready')
+      await store.loadPlacementRuns()
+    })
+  } finally {
+    planningId.value = null
+  }
+}
+
+async function onReview(run) {
+  await guard(async () => {
+    reviewRun.value = await store.getPlacementRun(run.id)
+    reviewOpen.value = true
+  })
+}
+
+function ask(title, message, action) {
+  confirmTitle.value = title
+  confirmMessage.value = message
+  confirmAction = action
+  confirmOpen.value = true
+}
+
+function onApply(run) {
+  ask(
+    'Move these files?',
+    `${run.planned_count.toLocaleString()} files move into their artist's `
+    + 'folder. The record and the file move together, and this run can be '
+    + 'put back afterwards.',
+    async () => {
+      await store.applyPlacement(run.id)
+      toast('Moving files — this runs in the background')
+      await store.loadPlacementRuns()
+    },
+  )
+}
+
+function onRevert(run) {
+  ask(
+    'Put these files back?',
+    `${run.moved_count.toLocaleString()} files return to where they were `
+    + 'before this run.',
+    async () => {
+      await store.revertPlacement(run.id)
+      toast('Putting files back — this runs in the background')
+      await store.loadPlacementRuns()
+    },
+  )
+}
+
+async function onConfirmed() {
+  const action = confirmAction
+  confirmAction = null
+  if (action) await guard(action)
+}
+
+onMounted(async () => {
+  await guard(async () => {
+    artists.value = await store.loadArtistNames()
+    await store.loadPlacementRuns()
+  })
+  // Runs change state in a worker, so the table needs refreshing. The rows
+  // are server-side, which is why no localStorage resurfacing is needed here
+  // (useMaintenanceTask's job) — a reload or another machine sees the same
+  // state because the state IS the row.
+  poll = setInterval(() => { store.loadPlacementRuns().catch(() => {}) }, 5000)
+})
+
+onUnmounted(() => { if (poll) clearInterval(poll) })
+</script>
diff --git a/frontend/src/stores/cleanup.js b/frontend/src/stores/cleanup.js
index 82813a2..73b17d7 100644
--- a/frontend/src/stores/cleanup.js
+++ b/frontend/src/stores/cleanup.js
@@ -71,10 +71,66 @@ export const useCleanupStore = defineStore('cleanup', () => {
     return await api.post(`/api/cleanup/audit/${id}/cancel`)
   }
 
+  // --- placement reconciler (milestone #421) --------------------------------
+  //
+  // Runs are server-side rows, so the DATABASE is the durable state here —
+  // no localStorage resurfacing (useMaintenanceTask) is needed. Reload the
+  // page, open it on another machine, and the run and its status are simply
+  // there. That also means a plan survives being walked away from for a day.
+
+  const placementRuns = ref([])
+  const layout = ref(null)
+
+  // The survey: which rows sit outside their artist's directory. check_disk
+  // additionally stats every destination (collisions, missing sources) and
+  // costs one stat per misplaced row over NFS, so it is opt-in.
+  async function loadLayout(checkDisk = false) {
+    layout.value = await api.get('/api/cleanup/layout', {
+      params: checkDisk ? { check_disk: 1 } : {},
+    })
+    return layout.value
+  }
+
+  // id -> name, so a run row can say "Conto" instead of "#47". The runs
+  // endpoint carries artist_id alone: the name belongs to the artist, and
+  // denormalising it into every run would go stale the moment one is renamed.
+  async function loadArtistNames() {
+    const rows = await api.get('/api/artists/names')
+    return Object.fromEntries((rows || []).map(a => [a.id, a.name]))
+  }
+
+  async function loadPlacementRuns(limit = 25) {
+    const body = await api.get('/api/cleanup/placement/runs', { params: { limit } })
+    placementRuns.value = body.runs || []
+    return placementRuns.value
+  }
+
+  // Detail carries `moves` — the plan the operator reads before agreeing.
+  async function getPlacementRun(id) {
+    return await api.get(`/api/cleanup/placement/runs/${id}`)
+  }
+
+  async function planPlacement(artistId = null) {
+    return await api.post('/api/cleanup/placement/plan', {
+      body: artistId === null ? {} : { artist_id: artistId },
+    })
+  }
+
+  async function applyPlacement(id) {
+    return await api.post(`/api/cleanup/placement/runs/${id}/apply`)
+  }
+
+  async function revertPlacement(id) {
+    return await api.post(`/api/cleanup/placement/runs/${id}/revert`)
+  }
+
   return {
     defaults, recentRuns,
     loadDefaults,
     previewMinDim, deleteMinDim,
     startAudit, getAudit, loadHistory, latestAuditForRule, applyAudit, cancelAudit,
+    placementRuns, layout,
+    loadLayout, loadArtistNames, loadPlacementRuns, getPlacementRun,
+    planPlacement, applyPlacement, revertPlacement,
   }
 })
diff --git a/frontend/test/placement.spec.js b/frontend/test/placement.spec.js
new file mode 100644
index 0000000..af5fc1a
--- /dev/null
+++ b/frontend/test/placement.spec.js
@@ -0,0 +1,111 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useCleanupStore } from '../src/stores/cleanup.js'
+import { stubFetch } from './stubFetch.js'
+
+
+describe('placement reconciler store (milestone #421)', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+  afterEach(() => vi.restoreAllMocks())
+
+  it('loadLayout leaves the disk check off by default', async () => {
+    const s = useCleanupStore()
+    let seen = ''
+    stubFetch((url) => {
+      seen = url
+      return { status: 200, body: { total_rows: 10, misplaced_rows: 2, artists: [] } }
+    })
+    await s.loadLayout()
+    // One stat per misplaced row over NFS is the cost; it must be opt-in.
+    expect(seen).not.toContain('check_disk')
+    expect(s.layout.misplaced_rows).toBe(2)
+  })
+
+  it('loadLayout asks for the disk check when requested', async () => {
+    const s = useCleanupStore()
+    let seen = ''
+    stubFetch((url) => {
+      seen = url
+      return { status: 200, body: { total_rows: 0, misplaced_rows: 0, artists: [] } }
+    })
+    await s.loadLayout(true)
+    expect(seen).toContain('check_disk=1')
+  })
+
+  it('planPlacement scopes to an artist when given one', async () => {
+    const s = useCleanupStore()
+    let sent = null
+    stubFetch((url, init) => {
+      sent = JSON.parse(init.body)
+      return { status: 202, body: { status: 'dispatched' } }
+    })
+    await s.planPlacement(47)
+    expect(sent).toEqual({ artist_id: 47 })
+  })
+
+  it('planPlacement sends no scope for the whole library', async () => {
+    const s = useCleanupStore()
+    let sent = null
+    stubFetch((url, init) => {
+      sent = JSON.parse(init.body)
+      return { status: 202, body: { status: 'dispatched' } }
+    })
+    await s.planPlacement()
+    // Not `{artist_id: null}` — the endpoint rejects a non-integer, and an
+    // absent key is how "whole library" is spelled.
+    expect(sent).toEqual({})
+  })
+
+  it('loadPlacementRuns keeps the rows for the table', async () => {
+    const s = useCleanupStore()
+    stubFetch(() => ({
+      status: 200,
+      body: { runs: [{ id: 3, status: 'ready', planned_count: 12 }] },
+    }))
+    await s.loadPlacementRuns()
+    expect(s.placementRuns).toHaveLength(1)
+    expect(s.placementRuns[0].status).toBe('ready')
+  })
+
+  it('getPlacementRun carries the moves — it is the preview', async () => {
+    const s = useCleanupStore()
+    stubFetch(() => ({
+      status: 200,
+      body: {
+        id: 3, status: 'ready', planned_count: 1,
+        moves: [{ image_id: 9, from: '/images/Conto/x.png', to: '/images/conto/x.png' }],
+      },
+    }))
+    const run = await s.getPlacementRun(3)
+    expect(run.moves[0].from).toBe('/images/Conto/x.png')
+    expect(run.moves[0].to).toBe('/images/conto/x.png')
+  })
+
+  it('loadArtistNames maps id to name for the run rows', async () => {
+    const s = useCleanupStore()
+    stubFetch(() => ({
+      status: 200,
+      body: [{ id: 47, name: 'Conto', slug: 'conto' }],
+    }))
+    expect(await s.loadArtistNames()).toEqual({ 47: 'Conto' })
+  })
+
+  it('loadArtistNames survives an empty roster', async () => {
+    const s = useCleanupStore()
+    stubFetch(() => ({ status: 200, body: [] }))
+    expect(await s.loadArtistNames()).toEqual({})
+  })
+
+  it('applyPlacement and revertPlacement post to their own run', async () => {
+    const s = useCleanupStore()
+    const urls = []
+    stubFetch((url) => {
+      urls.push(url)
+      return { status: 202, body: { status: 'dispatched' } }
+    })
+    await s.applyPlacement(5)
+    await s.revertPlacement(5)
+    expect(urls[0]).toContain('/api/cleanup/placement/runs/5/apply')
+    expect(urls[1]).toContain('/api/cleanup/placement/runs/5/revert')
+  })
+})
diff --git a/frontend/test/stubFetch.js b/frontend/test/stubFetch.js
new file mode 100644
index 0000000..80dc8fd
--- /dev/null
+++ b/frontend/test/stubFetch.js
@@ -0,0 +1,24 @@
+import { vi } from 'vitest'
+
+// The canonical fetch stub for store specs.
+//
+// `handler(url, init)` returns `{ status, body }`; body is JSON-encoded, and
+// `ok` is derived from the status so a store's error path can be exercised by
+// returning 4xx/5xx. Returns the vi.fn so a caller can assert on calls.
+//
+// Extracted 2026-09-21 from six specs carrying byte-identical copies
+// (adminStore, credentials, dbMaintenance, gallery, suggestions,
+// galleryRelatedStrip). Those still hold their own; migrate each the next
+// time it is touched rather than in one sweep.
+export function stubFetch (handler) {
+  globalThis.fetch = vi.fn(async (url, init) => {
+    const { status, body } = handler(url, init)
+    return {
+      ok: status >= 200 && status < 300,
+      status,
+      statusText: String(status),
+      text: async () => (body == null ? '' : JSON.stringify(body)),
+    }
+  })
+  return globalThis.fetch
+}
diff --git a/tests/test_api_placement.py b/tests/test_api_placement.py
new file mode 100644
index 0000000..3fd56b4
--- /dev/null
+++ b/tests/test_api_placement.py
@@ -0,0 +1,167 @@
+"""The placement reconciler's task + API surface (milestone #421, slice 3b).
+
+The move logic itself is covered in tests/test_library_layout.py; this module
+covers the wrapper — that the tasks are registered and routed, that the
+endpoints gate on run state, and that a list response stays small.
+"""
+
+import pytest
+from sqlalchemy import select
+
+import backend.app.tasks.library_placement  # noqa: F401 — register tasks
+from backend.app.celery_app import celery
+from backend.app.models import Artist, LibraryPlacementRun
+
+pytestmark = pytest.mark.integration
+
+_TASKS = (
+    "backend.app.tasks.library_placement.plan_placement",
+    "backend.app.tasks.library_placement.apply_placement",
+    "backend.app.tasks.library_placement.revert_placement",
+)
+
+
+@pytest.mark.parametrize("name", _TASKS)
+def test_placement_tasks_are_registered(name):
+    assert name in celery.tasks
+
+
+def test_placement_runs_on_the_long_maintenance_lane():
+    """33k renames must not sit in the quick lane, which is where the
+    self-healing sweeps live (the 2026-06-07 starvation)."""
+    routes = celery.conf.task_routes
+    assert routes["backend.app.tasks.library_placement.*"] == {
+        "queue": "maintenance_long"
+    }
+
+
+def _run(db, status="ready", moves=None, artist_id=None):
+    """Adds the row; the caller awaits the COMMIT.
+
+    Commit, not flush: the app under test runs on its own session and
+    connection, so a flush that stays inside this test's transaction is
+    invisible to the endpoint — the row simply is not there yet. Same reason
+    `_seed_runs` in test_api_system_backup commits."""
+    run = LibraryPlacementRun(
+        status=status, artist_id=artist_id, moves=moves or [],
+        planned_count=len(moves or []),
+    )
+    db.add(run)
+    return run
+
+
+@pytest.mark.asyncio
+async def test_runs_list_omits_the_moves(client, db):
+    """An applied whole-library run carries tens of thousands of entries.
+    Fine in Postgres, wrong in every list response."""
+    run = LibraryPlacementRun(
+        status="applied",
+        moves=[{"image_id": 1, "from": "/images/A/x.png", "to": "/images/a/x.png"}],
+        planned_count=1, moved_count=1,
+    )
+    db.add(run)
+    await db.commit()
+
+    resp = await client.get("/api/cleanup/placement/runs")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["runs"][0]["planned_count"] == 1
+    assert "moves" not in body["runs"][0]
+
+
+@pytest.mark.asyncio
+async def test_run_detail_carries_the_moves(client, db):
+    """The detail IS the preview the operator reads before agreeing."""
+    run = LibraryPlacementRun(
+        status="ready",
+        moves=[{"image_id": 7, "from": "/images/Conto/x.png", "to": "/images/conto/x.png"}],
+        planned_count=1,
+    )
+    db.add(run)
+    await db.commit()
+
+    resp = await client.get(f"/api/cleanup/placement/runs/{run.id}")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["moves"][0]["from"] == "/images/Conto/x.png"
+    assert body["moves"][0]["to"] == "/images/conto/x.png"
+
+
+@pytest.mark.asyncio
+async def test_run_detail_404s_for_an_unknown_run(client):
+    resp = await client.get("/api/cleanup/placement/runs/999999")
+    assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_plan_rejects_a_non_integer_artist(client):
+    resp = await client.post(
+        "/api/cleanup/placement/plan", json={"artist_id": "conto"},
+    )
+    assert resp.status_code == 400
+    assert (await resp.get_json())["error"] == "invalid_artist_id"
+
+
+@pytest.mark.asyncio
+async def test_plan_accepts_an_artist_scope(client, db, monkeypatch):
+    sent = {}
+    from backend.app.tasks import library_placement
+
+    monkeypatch.setattr(
+        library_placement.plan_placement, "delay",
+        lambda artist_id=None: sent.update(artist_id=artist_id),
+    )
+    artist = Artist(name="Conto", slug="conto")
+    db.add(artist)
+    await db.commit()
+
+    resp = await client.post(
+        "/api/cleanup/placement/plan", json={"artist_id": artist.id},
+    )
+    assert resp.status_code == 202
+    assert sent["artist_id"] == artist.id
+
+
+@pytest.mark.asyncio
+async def test_apply_refuses_a_run_that_is_not_ready(client, db):
+    """The gate is here as well as in the service — an applied run must not
+    be re-applied by a stray POST."""
+    run = _run(db, status="applied")
+    await db.commit()
+
+    resp = await client.post(f"/api/cleanup/placement/runs/{run.id}/apply")
+    assert resp.status_code == 400
+    assert (await resp.get_json())["error"] == "not_ready"
+
+
+@pytest.mark.asyncio
+async def test_revert_refuses_a_run_that_was_never_applied(client, db):
+    run = _run(db, status="ready")
+    await db.commit()
+
+    resp = await client.post(f"/api/cleanup/placement/runs/{run.id}/revert")
+    assert resp.status_code == 400
+    assert (await resp.get_json())["error"] == "not_applied"
+
+
+@pytest.mark.asyncio
+async def test_apply_dispatches_for_a_ready_run(client, db, monkeypatch):
+    sent = {}
+    from backend.app.tasks import library_placement
+
+    monkeypatch.setattr(
+        library_placement.apply_placement, "delay",
+        lambda run_id: sent.update(run_id=run_id),
+    )
+    run = _run(db, status="ready")
+    await db.commit()
+
+    resp = await client.post(f"/api/cleanup/placement/runs/{run.id}/apply")
+    assert resp.status_code == 202
+    assert sent["run_id"] == run.id
+    # Dispatch only — the endpoint must not have moved anything itself.
+    still = (await db.execute(
+        select(LibraryPlacementRun.status)
+        .where(LibraryPlacementRun.id == run.id)
+    )).scalar_one()
+    assert still == "ready"
diff --git a/tests/test_library_layout.py b/tests/test_library_layout.py
index 52cd88d..1594082 100644
--- a/tests/test_library_layout.py
+++ b/tests/test_library_layout.py
@@ -213,3 +213,191 @@ def test_survey_is_read_only(db_sync, tmp_path):
     assert db_sync.execute(
         select(ImageRecord.path).where(ImageRecord.id == rec.id)
     ).scalar_one() == before
+
+
+# --- plan / apply / revert (#4246) ------------------------------------------
+
+
+def _staged(db, tmp_path, slug, stray, name="x.png", n=100):
+    """An artist with one file sitting in `stray`'s directory."""
+    artist = _artist(db, slug.title(), slug)
+    src = tmp_path / stray / name
+    src.parent.mkdir(parents=True, exist_ok=True)
+    src.write_bytes(b"pixels")
+    rec = _image(db, str(src), artist, n)
+    return artist, rec, src
+
+
+@pytest.mark.integration
+def test_plan_records_where_each_file_came_from(db_sync, tmp_path):
+    from backend.app.services.library_layout import plan_placement
+
+    _, rec, src = _staged(db_sync, tmp_path, "conto", "Conto", n=20)
+    run = plan_placement(db_sync, tmp_path)
+
+    assert run.status == "ready"
+    assert run.planned_count == 1
+    assert run.moves == [{
+        "image_id": rec.id,
+        "from": str(src),
+        "to": str(tmp_path / "conto" / "x.png"),
+    }]
+    # Planning touches nothing.
+    assert src.exists()
+    assert db_sync.get(ImageRecord, rec.id).path == str(src)
+
+
+@pytest.mark.integration
+def test_apply_moves_file_and_row_together(db_sync, tmp_path):
+    from backend.app.services.library_layout import apply_run, plan_placement
+
+    _, rec, src = _staged(db_sync, tmp_path, "conto", "Conto", n=21)
+    run = apply_run(db_sync, plan_placement(db_sync, tmp_path))
+
+    dest = tmp_path / "conto" / "x.png"
+    assert run.status == "applied"
+    assert run.moved_count == 1 and run.refused_count == 0
+    assert dest.exists() and not src.exists()
+    db_sync.expire_all()
+    assert db_sync.get(ImageRecord, rec.id).path == str(dest)
+
+
+@pytest.mark.integration
+def test_revert_puts_it_back(db_sync, tmp_path):
+    """The whole reason `from` is retained: do one artist, look, undo."""
+    from backend.app.services.library_layout import (
+        apply_run,
+        plan_placement,
+        revert_run,
+    )
+
+    _, rec, src = _staged(db_sync, tmp_path, "conto", "Conto", n=22)
+    run = revert_run(db_sync, apply_run(db_sync, plan_placement(db_sync, tmp_path)))
+
+    assert run.status == "reverted"
+    assert src.exists()
+    assert not (tmp_path / "conto" / "x.png").exists()
+    db_sync.expire_all()
+    assert db_sync.get(ImageRecord, rec.id).path == str(src)
+
+
+@pytest.mark.integration
+def test_apply_refuses_a_row_that_moved_since_planning(db_sync, tmp_path):
+    """A supersede or an earlier run can rewrite a path between plan and
+    apply. The stale entry is declined, not forced."""
+    from backend.app.services.library_layout import apply_run, plan_placement
+
+    _, rec, src = _staged(db_sync, tmp_path, "conto", "Conto", n=23)
+    run = plan_placement(db_sync, tmp_path)
+
+    elsewhere = tmp_path / "conto" / "already-here.png"
+    elsewhere.parent.mkdir(parents=True, exist_ok=True)
+    elsewhere.write_bytes(b"pixels")
+    rec.path = str(elsewhere)
+    db_sync.flush()
+
+    run = apply_run(db_sync, run)
+
+    assert run.moved_count == 0 and run.refused_count == 1
+    assert run.refusals[0]["reason"] == "row moved since planning"
+    assert src.exists()  # untouched
+
+
+@pytest.mark.integration
+def test_apply_never_overwrites_an_occupied_destination(db_sync, tmp_path):
+    from backend.app.services.library_layout import apply_run, plan_placement
+
+    _, rec, src = _staged(db_sync, tmp_path, "conto", "Conto", n=24)
+    run = plan_placement(db_sync, tmp_path)
+
+    squatter = tmp_path / "conto" / "x.png"
+    squatter.parent.mkdir(parents=True, exist_ok=True)
+    squatter.write_bytes(b"someone else")
+
+    run = apply_run(db_sync, run)
+
+    assert run.refused_count == 1
+    assert run.refusals[0]["reason"] == "destination occupied"
+    assert squatter.read_bytes() == b"someone else"
+    db_sync.expire_all()
+    assert db_sync.get(ImageRecord, rec.id).path == str(src)
+
+
+@pytest.mark.integration
+def test_apply_leaves_the_row_alone_when_the_source_is_gone(db_sync, tmp_path):
+    from backend.app.services.library_layout import apply_run, plan_placement
+
+    _, rec, src = _staged(db_sync, tmp_path, "conto", "Conto", n=25)
+    run = plan_placement(db_sync, tmp_path)
+    src.unlink()
+
+    run = apply_run(db_sync, run)
+
+    assert run.refusals[0]["reason"] == "source missing"
+    db_sync.expire_all()
+    # The row still points at the missing file rather than at a file that
+    # was never created — a broken row is recoverable, a lying one is not.
+    assert db_sync.get(ImageRecord, rec.id).path == str(src)
+
+
+@pytest.mark.integration
+def test_plan_scopes_to_one_artist(db_sync, tmp_path):
+    """Per-artist scope is what makes this incremental instead of one
+    irreversible sweep."""
+    from backend.app.services.library_layout import plan_placement
+
+    conto, _, _ = _staged(db_sync, tmp_path, "conto", "Conto", n=26)
+    _staged(db_sync, tmp_path, "maewix", "Maewix", name="y.png", n=27)
+
+    run = plan_placement(db_sync, tmp_path, artist_id=conto.id)
+
+    assert run.planned_count == 1
+    assert run.artist_id == conto.id
+    assert "Conto" in run.moves[0]["from"]
+
+
+@pytest.mark.integration
+def test_plan_skips_both_rows_when_two_want_one_destination(db_sync, tmp_path):
+    """Which of two colliding rows 'wins' is not this sweep's call."""
+    from backend.app.services.library_layout import plan_placement
+
+    artist = _artist(db_sync, "Sticky", "sticky")
+    for stray, n in (("StickySpoodge", 28), ("Stickyspoodge", 29)):
+        p = tmp_path / stray / "dup.png"
+        p.parent.mkdir(parents=True, exist_ok=True)
+        p.write_bytes(b"pixels")
+        _image(db_sync, str(p), artist, n)
+
+    run = plan_placement(db_sync, tmp_path)
+
+    assert run.planned_count == 0
+
+
+@pytest.mark.integration
+def test_thumbnails_do_not_move(db_sync, tmp_path):
+    """Thumbs are sha-addressed (`thumbs/<xx>/<sha>.jpg`), not path-keyed, so
+    a placement move must not touch them. Pinned so nobody 'fixes' it."""
+    from backend.app.services.library_layout import apply_run, plan_placement
+
+    artist, rec, _ = _staged(db_sync, tmp_path, "conto", "Conto", n=30)
+    thumb = tmp_path / "thumbs" / "ab" / "abc.jpg"
+    thumb.parent.mkdir(parents=True, exist_ok=True)
+    thumb.write_bytes(b"thumb")
+    rec.thumbnail_path = str(thumb)
+    db_sync.flush()
+
+    apply_run(db_sync, plan_placement(db_sync, tmp_path))
+
+    db_sync.expire_all()
+    assert thumb.exists()
+    assert db_sync.get(ImageRecord, rec.id).thumbnail_path == str(thumb)
+
+
+@pytest.mark.integration
+def test_apply_refuses_a_run_that_is_not_ready(db_sync, tmp_path):
+    from backend.app.services.library_layout import apply_run, plan_placement
+
+    _staged(db_sync, tmp_path, "conto", "Conto", n=31)
+    run = apply_run(db_sync, plan_placement(db_sync, tmp_path))
+    with pytest.raises(ValueError):
+        apply_run(db_sync, run)