refactor(sync): the empty-answer rule for a mirror is one function, vanished()

The YouTube playlist guard from the previous commit put sync/playlists.py over its size ceiling;
the rule now lives beside should_prune as vanished(existing, listed), which the mirror calls in
place of its own list. The pyright baseline also drops by one (187), from the plex/overlay split.

backend/app/sync/playlists.py

@@ -22,7 +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.subscriptions import vanished
 from app.sync.videos import apply_video_details, parse_dt
 from app.youtube.client import YouTubeClient, YouTubeError
 
@@ -338,18 +338,10 @@ def pull_user_playlists(db: Session, user: User) -> dict:
                     Playlist.source == "local",
                     Playlist.yt_playlist_id.is_not(None),
                 )
-            )
-            .scalars()
-            .all()
-        )
-        # Drop mirrors whose YouTube playlist no longer exists — locked first, as every item write.
-        # 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 []
+            ).scalars()
         )
+        # Drop vanished mirrors (none on an empty answer) — locked first, as every item write.
+        gone = vanished(existing, yt_ids)
         if gone:
             lock_playlists(db, *(pl.id for pl in gone))
         for pl in gone: