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/plex/client.py

@@ -78,9 +78,10 @@ class PlexClient:
     # --- Catalog mirror (P1) ---
     def section_items(
         self, key: str, item_type: int, start: int = 0, size: int = 200
-    ) -> tuple[list[dict], int]:
+    ) -> tuple[list[dict], int | None]:
         """A page of a section's top-level items. type 1 = movie, 2 = show. Returns
-        (items, total_size)."""
+        (items, total_size); the total is None when the answer carries none, so a container that
+        says "0" (a genuinely empty section) stays distinguishable from one that says nothing."""
         mc = self._get(
             f"/library/sections/{key}/all",
             params={
@@ -93,7 +94,8 @@ class PlexClient:
                 "X-Plex-Container-Size": size,
             },
         )
-        return mc.get("Metadata", []) or [], int(mc.get("totalSize", mc.get("size", 0)) or 0)
+        total = mc.get("totalSize", mc.get("size"))
+        return mc.get("Metadata", []) or [], None if total is None else int(total)
 
     def children(self, rating_key: str) -> list[dict]:
         """Direct children (show → seasons, season → episodes)."""