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/subscriptions.py

@@ -1,6 +1,6 @@
 """Import the authenticated user's YouTube subscriptions and channel metadata."""
 
-from collections.abc import Iterable
+from collections.abc import Collection, Iterable
 from datetime import datetime, timedelta, timezone
 
 from sqlalchemy import or_, select
@@ -119,6 +119,15 @@ def should_prune(fetched_count: int, existing_count: int) -> bool:
     return fetched_count > 0 or existing_count == 0
 
 
+def vanished[K, V](existing: dict[K, V], listed: Collection[K]) -> list[V]:
+    """The local mirrors whose key the remote listing no longer has — none at all when the listing
+    is an empty answer that contradicts them (`should_prune`). The YouTube playlist mirrors use it:
+    without it, one blip dropped every mirror (E32 S2)."""
+    if not should_prune(len(listed), len(existing)):
+        return []
+    return [row for key, row in existing.items() if key not in listed]
+
+
 def import_subscriptions(db: Session, user: User) -> dict:
     """Pull all subscriptions for `user`, upsert channels + subscription links, prune
     channels the user has unsubscribed from on YouTube, and fetch channel details for