fix(sync): an empty answer the remote's own count contradicts is not believed, one level down

E32 review round 1 (B1, B2, B3, B8).

- Plex watch reconcile: a section the server SAYS is empty (totalSize 0, nothing listed) is a
  complete listing. It read as "cut short" forever, so one empty library stopped un-watches from
  ever syncing. A listing with no total at all stays incomplete (the client now returns None for
  it instead of 0), and a whole scan that listed nothing still clears nothing. Tests now cover
  completeness across two sections, both orders.
- Plex collections: no members from a collection whose listing counts some (childCount) keeps
  the membership we hold, in the full sync and in the targeted resync after a write.
- YouTube playlists: no items for a playlist whose listing counts some (itemCount) keeps the
  mirror in the bulk pull, and is refused as retryable in "reset to YouTube" and in the push
  plan/merge, where it read as "YouTube deleted them all". The rule sits beside vanished().
- A playlist deleted on YouTube answered "reconnect your account"; it now says it is gone
  (errors.youtube.gone, HU+EN).
- The pure 3-way merge moved to sync/playlist_merge.py; playlists.py is back under 800 lines and
  off the size list.

backend/app/sync/subscriptions.py

@@ -9,7 +9,7 @@ from sqlalchemy.orm import Session
 from app import logs
 from app.models import Channel, Subscription, User
 from app.utils import normalize_description
-from app.youtube.client import YouTubeClient, best_thumbnail
+from app.youtube.client import YouTubeClient, YouTubeError, best_thumbnail
 
 log = logs.get_logger(__name__)
 
@@ -115,7 +115,8 @@ def should_prune(fetched_count: int, existing_count: int) -> bool:
     has — taking each row's `deep_requested` flag with it — in a single run. Someone who genuinely
     unsubscribed from everything simply keeps their rows until a sync that returns something.
 
-    Pure so it can be tested without a database; `import_subscriptions` is the only caller."""
+    Pure so it can be tested without a database; its callers are `import_subscriptions` and, through
+    `vanished`, every mirror that drops what the remote no longer lists."""
     return fetched_count > 0 or existing_count == 0
 
 
@@ -128,6 +129,23 @@ def vanished[K, V](existing: dict[K, V], listed: Collection[K]) -> list[V]:
     return [row for key, row in existing.items() if key not in listed]
 
 
+def blank_listing(listed: Collection, count: object) -> bool:
+    """The same rule one level down: no members from a container whose own listing counts some (a
+    playlist's `itemCount`) is a blip, not a list emptied. Replacing or merging on it deleted every
+    mirrored item and reset the merge baseline (E32 review)."""
+    return not listed and isinstance(count, int) and count > 0
+
+
+def refuse_blank_listing(listed: Collection, count: object) -> None:
+    """`blank_listing` for a user-started pull or push: refused as retryable ("try again")."""
+    if blank_listing(listed, count):
+        raise YouTubeError(
+            "YouTube listed no items for a playlist it counts as non-empty",
+            reason="emptyAnswer",
+            path="playlistItems",
+        )
+
+
 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