feat: placement reconciler — plan, apply, revert (4246, slice 3a)

Milestone #421 step 3, reframed on the operator's steer: not a one-off
migration but the system that keeps the tree true. The 33,789 misplaced rows
the survey found are just its first run.

The placement half was already done, verified by reading each writer rather
than assuming: downloads have always written `<root>/<slug>/<platform>/`
(gallery_dl.py:523), attach_in_place leaves files where the downloader put
them, and `_copy_to_library` / `_supersede` became canonical in #4244. So
nothing is written off-canon today; what remains is the backlog and a standing
check for future drift.

`LibraryPlacementRun` (migration 0099) holds the plan as JSONB, and that one
structure does three jobs: it is the PREVIEW the operator reads, the list the
APPLY executes (rather than re-deriving the set, so the two cannot disagree),
and — because `from` is retained — the UNDO.

The undo is the point. It makes a 33,789-file operation something to do one
artist at a time, look at in the gallery, and reverse if it reads wrong. That
settles whether artist_id or the folder held the truth (spike #4257) by doing
rather than by arguing it from a 50-row sample.

An applied run is therefore HISTORY, not state — lesson #4226's trap, since
it is the only record of where those files used to be. The model and the
migration both say so: any future retention here may prune ready/cancelled/
error runs, never an applied one.

Everything fails closed. The apply re-checks each row against what the plan
recorded — source still there, destination still free, row still pointing
where the plan said — because a download or a supersede can land in between.
A refusal is recorded with its reason and the run continues; one stale row is
not a reason to abandon the other 33,788. The row is updated only after its
rename lands, so a failed move can never leave `path` naming a file that is
not there.

Writing the collision test caught the code disagreeing with its own comment:
it claimed the first of two rows wanting one destination and skipped the
second, silently picking a winner by iteration order. Now it counts first and
filters after, so genuinely neither is planned.

Thumbnails are sha-addressed, not path-keyed, so they do not move — pinned by
a test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

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",