feat(admin): prune_low_confidence_predictions backfill task + UI (#764)

The one-time backfill that actually shrinks the DB: drops stored
tagger_predictions entries below ml_settings.tagger_store_floor from every
image_record row, and clamps any allowlist min_confidence below the floor up
to it. Keep predicate (confidence >= floor) mirrors Tagger.infer's store gate
so backfilled rows match new imports. Keyset by id ASC, idempotent,
self-resumes on the soft time limit; runs on the maintenance_long lane.

pg_dump copies live data only, so this alone fixes the #739 backup timeout —
the reclaim (VACUUM FULL / pg_repack on image_record) is a separate, optional
disk-return step, brief because post-prune the live data is tiny.

- admin.prune_low_confidence_predictions_task + POST /api/admin/maintenance/prune-predictions
- PrunePredictionsCard in the Maintenance panel (shows the current floor)
- tests: registration + prune-keeps->=floor/drops-<floor + allowlist clamp

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

backend/app/api/admin.py

@@ -348,3 +348,15 @@ async def trigger_reextract_archives():
 
     async_result = reextract_archive_attachments_task.delay()
     return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
+@admin_bp.route("/maintenance/prune-predictions", methods=["POST"])
+async def trigger_prune_predictions():
+    """Operator-triggered #764 backfill: drop stored tagger predictions below
+    the current ml_settings.tagger_store_floor and clamp allowlist thresholds
+    up to it. Shrinks image_record's TOAST (~100 GB of sub-0.70 scores).
+    Idempotent + self-resuming; runs on the maintenance_long lane."""
+    from ..tasks.admin import prune_low_confidence_predictions_task
+
+    async_result = prune_low_confidence_predictions_task.delay()
+    return jsonify({"task_id": async_result.id, "status": "queued"}), 202