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_phash_dedup.py

@@ -11,6 +11,7 @@ from PIL import Image
 from sqlalchemy import func, select
 
 from backend.app.models import (
+    ImagePrediction,
     ImageProvenance,
     ImageRecord,
     ImportSettings,
@@ -118,7 +119,11 @@ def test_smaller_existing_is_superseded(importer, import_layout):
             image_record_id=eid, tag_id=tag.id, source="manual"
         )
     )
-    old.tagger_predictions = {"x": 1}
+    importer.session.add(
+        ImagePrediction(
+            image_record_id=eid, raw_name="x", category="general", score=0.9
+        )
+    )
     old.siglip_embedding = [0.0] * 1152
     old.integrity_status = "ok"
     importer.session.commit()
@@ -136,7 +141,11 @@ def test_smaller_existing_is_superseded(importer, import_layout):
     assert row.path != old_path
     assert row.phash is not None
     assert row.integrity_status == "unknown"
-    assert row.tagger_predictions is None
+    # #768: re-import clears the normalized predictions too
+    assert importer.session.execute(
+        select(func.count()).select_from(ImagePrediction)
+        .where(ImagePrediction.image_record_id == eid)
+    ).scalar_one() == 0
     assert row.siglip_embedding is None
     linked = importer.session.execute(
         select(image_tag.c.tag_id).where(