feat(integrity): structural verification + supersede-on-replace pipeline

Adds per-image integrity tracking so corrupt files are detected, excluded
from random/showcase/ML/suggestion paths, and recoverable by dropping a
fresh copy in /import — closing the gap that surfaced as the WD14
'6 bytes not processed' OSError.

Schema (migration l26042501)
- image_record.integrity_status: unknown | ok | truncated | unreadable | missing
- image_record.integrity_checked_at: timestamptz
- partial index on status <> 'ok' for cheap report/filter queries

Verifier
- app/services/integrity.py: verify_path() dispatches by extension
- PIL two-stage (verify + load with LOAD_TRUNCATED_IMAGES disabled)
- ffprobe for video, zipfile.testzip for archives
- Truncation-vs-unreadable distinction via PIL message hints

Pipeline
- verify_media_integrity Celery task: per-image, idempotent
- verify_unverified_images sweep: only_unknown by default, skips
  paths in active import tasks
- Hooked into the end of import_media_file (new + archive paths) and
  the supersede branch
- supersede_image() resets status to 'unknown' so the post-supersede
  verify writes a fresh truth
- Supersede-on-replace: a fresh /import/<artist>/<filename> matching
  a flagged-corrupt record routes through _supersede_existing,
  preserving tags/series/embeddings

Exclusions
- /, /api/random-images, tag_and_embed, ml.backfill enqueue, and
  get_suggestions all filter integrity_status IN ('ok', 'unknown') so
  flagged rows don't poison the gallery, ML, or suggestion math.
  'unknown' is treated as healthy so post-migration data stays visible
  until the sweep runs.

UI / report
- Settings -> Maintenance: 'Verify unknown' + 'Force re-verify all'
- GET /api/integrity/failed (paginated list of flagged rows)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/utils/image_importer.py

@@ -252,6 +252,11 @@ def supersede_image(old_record: "ImageRecord", new_filepath: str, new_thumb_path
     old_record.width = new_metadata["width"]
     old_record.height = new_metadata["height"]
     old_record.format = new_metadata["format"]
+    # The bytes on disk just changed — the prior verification result is no
+    # longer authoritative. Reset to 'unknown' so the post-supersede verify
+    # hook (or the next sweep) re-records the truth.
+    old_record.integrity_status = 'unknown'
+    old_record.integrity_checked_at = None
     # Keep the earlier taken_at date (prefer original date)
     if new_metadata.get("taken_at") and old_record.taken_at:
         if new_metadata["taken_at"] < old_record.taken_at: