fix(sync): an empty answer no longer empties a mirror; maintenance checks each video once a run

The catalog prune and the subscription import have long refused to reconcile against an empty
listing that contradicts what we hold (should_prune: a 200 with no items is a blip far more often
than a user who removed everything). Two mirrors did not: the YouTube playlist mirrors (dropped
every mirror) and the Plex collections (dropped every collection with its user-set `editable`
flag, and every member's keys — the guard landed in plex/sync.py with the previous commit). Both
now hold the same rule; test_empty_listing_guards covers each way (seen red).

sync/maintenance (19% covered) gets a lifecycle test: recover, grace, delete + one notification
per user, live stream kept, abandoned premiere. It found the rolling revalidation re-fetching the
same videos until `checked` reached the batch (15000 by default) whenever fewer videos were
available — quota spent on answers it already had; a run now skips what it has checked.

backend/app/sync/maintenance.py

@@ -22,7 +22,7 @@ backfill reserve as the other jobs so interactive syncs never starve.
 
 from datetime import timedelta
 
-from sqlalchemy import select
+from sqlalchemy import or_, select
 from sqlalchemy.orm import Session
 
 from app import logs, progress, quota, sysconfig
@@ -190,6 +190,10 @@ def _revalidate_rolling(db: Session, yt: YouTubeClient) -> dict:
             db.execute(
                 select(Video)
                 .where(Video.unavailable_since.is_(None))
+                # Not one this run already checked: with fewer available videos than the batch, the
+                # loop used to wrap around and re-fetch the same ones until `checked` reached the
+                # batch, spending quota on answers it already had (E32 S2).
+                .where(or_(Video.last_checked_at.is_(None), Video.last_checked_at < now))
                 # NULLS FIRST = never-checked first, then least-recently-checked; equivalent to the old
                 # `(last_checked_at IS NULL) DESC, last_checked_at ASC` but in the exact shape of the
                 # partial ix_videos_recheck_order (migration 0069, PERF-18) so it rides that index.