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_admin.py

@@ -42,75 +42,6 @@ def test_bulk_delete_images_task_registered():
     )
 
 
-def test_prune_low_confidence_predictions_task_registered():
-    assert (
-        "backend.app.tasks.admin.prune_low_confidence_predictions_task"
-        in celery.tasks
-    )
-
-
-def test_backfill_image_predictions_task_registered():
-    assert (
-        "backend.app.tasks.admin.backfill_image_predictions_task"
-        in celery.tasks
-    )
-
-
-@pytest.mark.asyncio
-async def test_prune_low_confidence_predictions(db_sync, tmp_path):
-    # #764: drop stored tagger predictions below the store floor (default
-    # 0.70) and clamp allowlist thresholds up to it.
-    from backend.app.models import Tag, TagAllowlist, TagKind
-    from backend.app.tasks.admin import prune_low_confidence_predictions_task
-
-    f0 = tmp_path / "p0.jpg"
-    f0.write_bytes(b"x")
-    img0 = ImageRecord(
-        path=str(f0), sha256=f"{0:064x}", size_bytes=10, mime="image/jpeg",
-        origin="imported_filesystem",
-        tagger_predictions={
-            "keep_high": {"category": "general", "confidence": 0.92},
-            "keep_edge": {"category": "general", "confidence": 0.70},
-            "drop_mid": {"category": "general", "confidence": 0.40},
-            "drop_tiny": {"category": "general", "confidence": 0.06},
-        },
-    )
-    db_sync.add(img0)
-    f1 = tmp_path / "p1.jpg"
-    f1.write_bytes(b"x")
-    img1 = ImageRecord(
-        path=str(f1), sha256=f"{1:064x}", size_bytes=10, mime="image/jpeg",
-        origin="imported_filesystem",
-        tagger_predictions={"only": {"category": "general", "confidence": 0.99}},
-    )
-    db_sync.add(img1)
-    tag = Tag(name="lowthr-tag", kind=TagKind.general)
-    db_sync.add(tag)
-    db_sync.flush()
-    db_sync.add(TagAllowlist(tag_id=tag.id, min_confidence=0.30))
-    db_sync.commit()
-    img0_id, img1_id, tag_id = img0.id, img1.id, tag.id
-
-    result = prune_low_confidence_predictions_task.delay().get()
-    assert result["floor"] == pytest.approx(0.70)
-    assert result["pruned"] == 1            # only img0 had sub-floor entries
-    assert result["allowlist_clamped"] == 1
-
-    db_sync.expire_all()
-    p0 = db_sync.execute(
-        select(ImageRecord.tagger_predictions).where(ImageRecord.id == img0_id)
-    ).scalar_one()
-    assert set(p0) == {"keep_high", "keep_edge"}   # >=0.70 kept, <0.70 dropped
-    p1 = db_sync.execute(
-        select(ImageRecord.tagger_predictions).where(ImageRecord.id == img1_id)
-    ).scalar_one()
-    assert set(p1) == {"only"}                      # already clean — untouched
-    clamped = db_sync.execute(
-        select(TagAllowlist.min_confidence).where(TagAllowlist.tag_id == tag_id)
-    ).scalar_one()
-    assert clamped == pytest.approx(0.70)
-
-
 # --- delete_artist_cascade_task -------------------------------------