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>

frontend/src/components/settings/MaintenancePanel.vue

@@ -12,6 +12,7 @@
       <ThumbnailBackfillCard />
     </div>
     <MLThresholdSliders class="mt-4" />
+    <BackfillPredictionsCard class="mt-4" />
     <PrunePredictionsCard class="mt-4" />
     <AllowlistTable class="mt-4" />
     <AliasTable class="mt-4" />
@@ -32,6 +33,7 @@ import MLBackfillCard from './MLBackfillCard.vue'
 import CentroidRecomputeCard from './CentroidRecomputeCard.vue'
 import ThumbnailBackfillCard from './ThumbnailBackfillCard.vue'
 import MLThresholdSliders from './MLThresholdSliders.vue'
+import BackfillPredictionsCard from './BackfillPredictionsCard.vue'
 import PrunePredictionsCard from './PrunePredictionsCard.vue'
 import AllowlistTable from './AllowlistTable.vue'
 import AliasTable from './AliasTable.vue'