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>

tests/test_tasks_ml.py

@@ -44,7 +44,7 @@ async def test_backfill_enqueues_missing(db, monkeypatch):
         path="/images/n.jpg", sha256="n" * 64, size_bytes=1,
         mime="image/jpeg", width=1, height=1,
         origin="imported_filesystem", integrity_status="unknown",
-        tagger_predictions=None, siglip_embedding=None,
+        siglip_embedding=None,
     )
     db.add(img)
     await db.commit()