feat(scheduler): admin-tunable maintenance re-validation batch size

Expose the maintenance job's rolling re-validation batch (videos re-checked per
run) as an admin control on the Scheduler dashboard, stored in app_state
(migration 0018; NULL = the env/config default). The job reads the effective
value each run via a state helper, clamped to a sane range. Trilingual.

backend/app/sync/maintenance.py

@@ -29,6 +29,7 @@ from app import progress, quota
 from app.config import settings
 from app.models import Playlist, PlaylistItem, Video, VideoState
 from app.notifications import create_notification
+from app.state import get_maintenance_batch
 from app.sync.runner import get_service_user
 from app.sync.videos import apply_video_details, parse_dt
 from app.youtube.client import YouTubeClient
@@ -189,7 +190,7 @@ def _recheck_flagged(db: Session, yt: YouTubeClient) -> dict:
 def _revalidate_rolling(db: Session, yt: YouTubeClient) -> dict:
     """Phase 2: re-check the least-recently-checked currently-available videos and flag any
     that have become unplayable (hidden immediately; deleted later if they don't recover)."""
-    batch = settings.maintenance_revalidate_batch
+    batch = get_maintenance_batch(db)  # admin override, else config default
     flagged_new = 0
     checked = 0
     now = _now()