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/models/__init__.py

@@ -22,7 +22,6 @@ 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
@@ -86,7 +85,6 @@ __all__ = [
     "ImportTask",
     "ImportSettings",
     "LibraryAuditRun",
-    "LibraryPlacementRun",
     "MembershipSync",
     "MLSettings",
     "HeadAutoApplyRun",