revert: remove the placement reconciler — it manufactured the problem it solved

Milestone #421 built a sweep that compared each image's `artist_id` to the
name of the directory holding its file, and called every mismatch a misplaced
image. It reported 33,789 of 63,605 as wrongly filed. That number described
the comparison, not the library.

What it actually was:

  32,475  (97.1%)  one artist's own folder, spelled differently
                   — Telepurte/ vs telepurte/. Same artist, same art.
     657  ( 2.0%)  loose at the images root
     328  ( 1.0%)  in a folder named after a different artist

And the 1% did not mean what the tool assumed either. `ImageProvenance`
records the post and source every file was downloaded from — the
authoritative answer, which the tool never consulted. Querying it for all 328:

    144  provenance agrees with the record  (move would be right)
     87  provenance agrees with the FOLDER  (the record is wrong; move wrong)
     53  provenance names SEVERAL artists   (no single correct folder)
     41  no provenance at all
      3  agrees with neither

So the sweep would have misfiled or arbitrarily picked for ~41% of the only
set it was really needed for. The system already knew where each file came
from; the tool inferred it from a column and a directory name instead.

Operator, 2026-09-21: *"the current system consistently records where items
are and where they came from this is just complicating something works and
doesn't need fixing."* Correct on both counts.

Removed: the service, the tasks, the model and migration 0099's table, the
/api/cleanup/layout and /placement/* endpoints, the Maintenance card and its
store actions, and the tests. 0100 drops the table (rule #22 — no legacy).

KEPT deliberately, per the operator:
- `utils.paths.canonical_subdir` — new filesystem imports derive their
  directory from the artist's slug, matching what the downloader always did.
  Not part of this tool; removing it would be churn that fixes nothing.
- The 327 files run 1 moved (InsoUwu/ -> insouwu/). Same artist either way,
  and the gallery renders them correctly.
- Everything from #4223 (three-gate dedup, 256-bit pHash) and #4234 (backup
  credential exclusion). Those fixed problems that were actually reported.

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

backend/app/celery_app.py

@@ -35,7 +35,6 @@ 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",
         ],
     )
@@ -63,8 +62,6 @@ 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