feat(maintenance): scheduled job to retire unplayable videos

Add a daily maintenance/validation job that detects videos which can't be played
anywhere and retires them safely. Two phases: re-check already-flagged videos
(recover if available again, else hard-delete once the grace period elapses,
cascading to states/playlist items), and a rolling re-validation of the
least-recently-checked currently-available videos that flags newly-unplayable
ones (hidden from the feed immediately via unavailable_since).

Detection is ~free: a video missing from the videos.list response is
deleted-or-private; an `upcoming` premiere >2 days past its scheduled start that
never went live is abandoned. A still-live broadcast is kept (legit 24/7 stream).
Enrichment now also fetches part=status to populate the status columns. Grace is
7 days for removed videos, none for abandoned. Before deleting, affected users
get one batched notification (never per-video). Interval is admin-tunable via the
Scheduler dashboard; batch size and grace are config. Quota-attributed to the
system and bounded by the same backfill reserve as the other jobs.

backend/app/routes/feed.py

@@ -154,6 +154,11 @@ def _filtered_query(
         state, and_(state.video_id == Video.id, state.user_id == user.id)
     )
 
+    # Hide videos the maintenance job has flagged as currently unplayable (deleted/private/
+    # abandoned). They're either deleted after a grace period or unhidden if they recover;
+    # either way they shouldn't show in any feed view meanwhile.
+    query = query.where(Video.unavailable_since.is_(None))
+
     if channel_id:
         query = query.where(Video.channel_id == channel_id)
     if min_duration is not None: