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/tasks/admin.py

@@ -122,6 +122,27 @@ def prune_missing_file_records_task(self) -> dict:
     return {"checked": checked, "missing": len(missing_ids), "deleted": deleted}
 
 
+@celery.task(
+    name="backend.app.tasks.admin.dedup_videos_task",
+    bind=True,
+    autoretry_for=(OperationalError, DBAPIError),
+    retry_backoff=15, retry_backoff_max=180, max_retries=1,
+    soft_time_limit=1800, time_limit=2400,  # 30 min / 40 min
+)
+def dedup_videos_task(self, dry_run: bool = False) -> dict:
+    """Tier-1 video dedup (#871): re-probe NULL-duration videos, cluster by
+    artist + duration + aspect, keep the highest-res copy per cluster. dry_run
+    returns the projection (groups/redundant/reclaimable bytes) WITHOUT deleting;
+    apply re-points each loser's post links to the keeper then deletes the
+    redundant records + files. Operator-triggered; the summary lands in
+    task_run.metadata (FC-3i) for the Maintenance card to surface."""
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        return cleanup_service.dedup_videos(
+            session, images_root=IMAGES_ROOT, dry_run=dry_run,
+        )
+
+
 @celery.task(
     name="backend.app.tasks.admin.bulk_delete_images_task",
     bind=True,