feat(maintenance): retroactive video-dedup action — preview + apply (#871)

Phase 2 of #871: clean up the duplicate videos already in the library (the #859
"same video from multiple sources" clutter). Import-time dedup (Phase 1) only
prevents NEW dups; this is the operator-triggered cleanup of existing ones.

cleanup_service.dedup_videos(dry_run):
- backfill_video_durations: re-probe NULL-duration videos (pre-#871 rows) so the
  existing library participates; idempotent (only NULL rows), writes a negative
  sentinel for un-probeable files so they're neither re-probed forever nor matched.
- find_video_dup_groups: cluster same-artist videos by duration (±tol) + aspect,
  anchored per cluster to bound the span (no chain drift); keeper = highest pixel
  area then bytes. Reuses the importer's _VIDEO_DUP_* tolerances.
- apply: re-point each loser's post links to the keeper (so no post loses the
  video) THEN delete the redundant records + files via delete_images (cascade).
  dry_run shares the same discovery predicate and returns the projection only
  (rule 93). Tags on a loser are NOT merged (noted; videos rarely hand-curated).

- dedup_videos_task (maintenance queue; summary → task_run.metadata).
- POST /maintenance/dedup-videos {dry_run} + GET /maintenance/task-result/<id> so
  the card shows the dry-run projection before the destructive apply.
- VideoDedupCard: Preview → shows groups/redundant/reclaimable, then Apply behind
  a confirm dialog. Mounted in the Maintenance panel.

Tests: dedup collapses + re-links the loser's post to the keeper + removes the
file; dry-run deletes nothing; distinct durations aren't grouped; task registered.
(Migration 0052 for duration_seconds already shipped with Phase 1.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/admin.py

@@ -361,3 +361,34 @@ async def trigger_prune_missing_files():
 
     async_result = prune_missing_file_records_task.delay()
     return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
+@admin_bp.route("/maintenance/dedup-videos", methods=["POST"])
+async def trigger_dedup_videos():
+    """Tier-1 video dedup (#871). Body {"dry_run": bool}: dry_run=true previews
+    what would be removed (groups / redundant count / reclaimable bytes) WITHOUT
+    deleting; dry_run=false applies it (re-link posts to the keeper, then delete
+    the redundant copies). Either way it first re-probes NULL-duration videos so
+    the existing library participates. Returns the Celery task id — poll
+    /maintenance/task-result/<id> for the summary."""
+    from ..tasks.admin import dedup_videos_task
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", True))  # default to the SAFE preview
+    async_result = dedup_videos_task.delay(dry_run=dry_run)
+    return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
+@admin_bp.route("/maintenance/task-result/<task_id>", methods=["GET"])
+async def maintenance_task_result(task_id: str):
+    """Poll a maintenance Celery task's result (the summary dict it returns).
+    Used by the video-dedup card to show the dry-run projection before apply."""
+    from ..celery_app import celery
+
+    res = celery.AsyncResult(task_id)
+    ready = res.ready()
+    return jsonify({
+        "ready": ready,
+        "successful": res.successful() if ready else None,
+        "result": res.result if (ready and res.successful()) else None,
+    })