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/playlists.py

@@ -22,6 +22,7 @@ from app import logs, progress, quota
 from app.auth import has_read_scope
 from app.models import Channel, Playlist, PlaylistItem, User, Video
 from app.sync.runner import token_users
+from app.sync.subscriptions import should_prune
 from app.sync.videos import apply_video_details, parse_dt
 from app.youtube.client import YouTubeClient, YouTubeError
 
@@ -342,7 +343,13 @@ def pull_user_playlists(db: Session, user: User) -> dict:
             .all()
         )
         # Drop mirrors whose YouTube playlist no longer exists — locked first, as every item write.
-        gone = [pl for ytid, pl in existing.items() if ytid not in yt_ids]
+        # Never on an empty answer that contradicts the mirrors we hold: the same 200-with-no-items
+        # blip `should_prune` guards the subscriptions against would drop every mirror (E32 S2).
+        gone = (
+            [pl for ytid, pl in existing.items() if ytid not in yt_ids]
+            if should_prune(len(yt_ids), len(existing))
+            else []
+        )
         if gone:
             lock_playlists(db, *(pl.id for pl in gone))
         for pl in gone: