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/youtube/client.py

@@ -327,7 +327,7 @@ class YouTubeClient:
             data = self._get(
                 "videos",
                 {
-                    "part": "snippet,contentDetails,statistics,topicDetails,liveStreamingDetails",
+                    "part": "snippet,contentDetails,statistics,topicDetails,liveStreamingDetails,status",
                     "id": ",".join(batch),
                     "maxResults": 50,
                 },