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_migration_0003.py

@@ -24,8 +24,10 @@ def test_new_tables_registered():
 
 def test_image_record_columns_renamed():
     cols = {c.name for c in ImageRecord.__table__.columns}
-    assert "tagger_predictions" in cols
+    # tagger_predictions (the renamed wd14_predictions) was later dropped in
+    # migration 0046 — predictions live in image_prediction now (#768).
     assert "tagger_model_version" in cols
+    assert "tagger_predictions" not in cols
     assert "wd14_predictions" not in cols
     assert "wd14_model_version" not in cols