fix(migration): make 0045 DDL-only; backfill image_prediction via batched task (#768)

The inline INSERT…SELECT backfill in migration 0045 wrapped the table
creation and a ~100 GB pass over image_record.tagger_predictions in one
transaction: nothing committed until the end, it was unmonitorable, and an
earlier MATERIALIZED-CTE form spilled the full 100 GB to temp on NFS. A
deploy got stuck on it for ~2h with image_prediction never appearing.

Split the concerns:
- 0045 now creates ONLY the table + indexes (instant DDL → web boots).
- New backend.app.tasks.admin.backfill_image_predictions_task copies the
  >= store-floor predictions from the JSON into image_prediction, batched by
  id window and committed per chunk: live progress, resumable (re-enqueues
  from the last committed id), idempotent (ON CONFLICT DO NOTHING). json_each
  stays in the DB executor streaming each window — no Python-side 100 GB load,
  no materialization.
- POST /api/admin/maintenance/backfill-predictions + a Maintenance-tab card
  to trigger the one-time run after upgrading.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_tasks_admin.py

@@ -49,6 +49,13 @@ def test_prune_low_confidence_predictions_task_registered():
     )
 
 
+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