fix(plex): the watch reconcile clears only what it walked, and doubts a mirrored 0

A library left out of the allowlist is never scanned, so its watch states were cleared on
every reconcile; clearing is now limited to the sections walked. A section we mirror items
from that says it holds 0 is not believed complete. The page size is no longer read as the
section total when totalSize is missing (measured: the server always sends totalSize on a
paged request, 0 for an empty one).

backend/app/plex/client.py

@@ -94,7 +94,9 @@ class PlexClient:
                 "X-Plex-Container-Size": size,
             },
         )
-        total = mc.get("totalSize", mc.get("size"))
+        # `totalSize` only: `size` is THIS page's count, and read as the total it made a single
+        # page look like the whole section — "complete" to a prune or a reconcile (E32 review).
+        total = mc.get("totalSize")
         return mc.get("Metadata", []) or [], None if total is None else int(total)
 
     def children(self, rating_key: str) -> list[dict]: