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/models/image_record.py

@@ -60,8 +60,10 @@ class ImageRecord(Base):
         ForeignKey("artist.id", ondelete="SET NULL"), nullable=True, index=True
     )
 
-    # ML fields (populated by FC-2's ml-worker)
-    tagger_predictions: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    # ML fields (populated by FC-2's ml-worker). Per-tag predictions live in the
+    # normalized image_prediction table (#768) — the tagger_predictions JSON
+    # column was dropped in migration 0046. tagger_model_version stays as the
+    # "has this been tagged / is it current?" signal the backfill sweep reads.
     tagger_model_version: Mapped[str | None] = mapped_column(String(128), nullable=True)
     # 1152 = SigLIP-so400m embedding dim. Swapping models in FC-2 may require
     # a column-width migration.

backend/app/models/tag_alias.py

@@ -1,6 +1,6 @@
 """TagAlias — maps a model's (name, category) prediction to the operator's
-canonical tag. Resolved at suggestion-read time so raw predictions stay
-unmolested in image_record.tagger_predictions.
+canonical tag. Resolved at suggestion-read time so the raw predictions stored
+in image_prediction stay unmolested.
 """
 
 from datetime import datetime