feat(ml): drop image_record.tagger_predictions — image_prediction is sole store (#768 step 3)

Read cutover verified in prod (suggestions + allowlist read image_prediction;
backfill complete at 908k rows / 51k images). Removes the old JSON column and
everything that fed it:

- ImageRecord.tagger_predictions column removed; migration 0046 DROPs it.
  tagger_model_version kept as the "tagged / current?" signal the backfill
  sweep reads (needs-tagging check switched to tagger_model_version IS NULL).
- tag_and_embed no longer dual-writes the JSON — image_prediction is the only
  write path.
- importer re-import reset drops the JSON line (image_prediction rows are
  already deleted on re-import).
- Retired the one-time #768 backfill task + the #764 prune task, their admin
  endpoints, and their Maintenance cards (Backfill/PrunePredictionsCard).
- Tests seed/assert via image_prediction; stale column refs removed.

Disk reclaim is NOT automatic: DROP COLUMN is a catalog change. Run
`VACUUM FULL image_record` off-hours afterward to return the ~100 GB to the OS
so DB backups go small (#739). image_prediction (~90 MB) stays in pg_dump — it's
the source of truth now.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/cleanup_service.py

@@ -574,7 +574,7 @@ def reset_content_tagging(session: Session, *, dry_run: bool = False) -> dict:
     can re-tag from scratch via the Camie auto-suggest.
 
     PRESERVED: fandom + series tags and their series_page ordering, plus every
-    image's image_record.tagger_predictions (untouched) so suggestions
+    image's image_prediction rows (untouched) so suggestions
     repopulate immediately. CASCADE on image_tag / tag_alias / tag_allowlist /
     tag_reference_embedding / tag_suggestion_rejection clears each deleted
     tag's applications + metadata. Tag.fandom_id is SET NULL, so deleting

backend/app/services/importer.py

@@ -1085,7 +1085,6 @@ class Importer:
         existing.height = height
         existing.thumbnail_path = None
         existing.integrity_status = "unknown"
-        existing.tagger_predictions = None
         existing.tagger_model_version = None
         existing.siglip_embedding = None
         existing.siglip_model_version = None

backend/app/services/ml/aliases.py

@@ -1,7 +1,7 @@
 """Alias resolution + CRUD.
 
 A tag_alias maps (model_name, model_category) -> canonical Tag. Resolution
-happens at suggestion-read time so raw tagger_predictions stay unmolested.
+happens at suggestion-read time so the raw image_prediction rows stay unmolested.
 """
 
 from collections.abc import Sequence