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>

backend/app/api/admin.py

@@ -360,3 +360,16 @@ async def trigger_prune_predictions():
 
     async_result = prune_low_confidence_predictions_task.delay()
     return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
+@admin_bp.route("/maintenance/backfill-predictions", methods=["POST"])
+async def trigger_backfill_predictions():
+    """Operator-triggered #768 backfill: copy stored tagger predictions from the
+    image_record.tagger_predictions JSON into the normalized image_prediction
+    table. Batched + resumable + idempotent; runs on the maintenance_long lane.
+    Run this once after deploying migration 0045 (which creates the empty table)
+    to populate predictions for the existing library."""
+    from ..tasks.admin import backfill_image_predictions_task
+
+    async_result = backfill_image_predictions_task.delay()
+    return jsonify({"task_id": async_result.id, "status": "queued"}), 202