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/sync.py

@@ -418,7 +418,10 @@ def _paginate(plex: PlexClient, key: str, item_type: int, progress: dict | None
     if progress is not None:
         # A truthy `total` we reached = a trustworthy complete listing. `total` unknown/zero-with-rows
         # (a Plex that doesn't report totalSize) is treated as NOT complete, so prune stays conservative.
+        # `total` itself is kept for a caller to whom a server that SAID 0 is news it can trust (the
+        # watch reconcile); an empty mirror or overlay must not read it so (E32 S2).
         progress["complete"] = bool(total) and start >= total
+        progress["total"] = total
 
 
 # The section listing exposes at most the top ~20 cast (Plex's own cap); a stored cast longer than this
@@ -671,9 +674,11 @@ def resync_collection(db: Session, plex: PlexClient, lib: PlexLibrary, rk: str)
     meta = plex.metadata(rk) or {}
     col = _upsert_collection(db, by_rating_key(db, PlexCollection, rk), rk, lib.id, meta)
     Model = PlexItem if lib.kind == "movie" else PlexShow
-    new_members = {
-        str(c.get("ratingKey")) for c in plex.collection_children(rk) if c.get("ratingKey")
-    }
+    children = _believed_children(plex, rk, meta)
+    if children is None:
+        db.flush()
+        return col
+    new_members = {str(c.get("ratingKey")) for c in children if c.get("ratingKey")}
     current = db.execute(select(Model).where(Model.collection_keys.contains([rk]))).scalars().all()
     for row in current:  # drop rk from rows that are no longer members
         if row.rating_key not in new_members:
@@ -686,6 +691,22 @@ def resync_collection(db: Session, plex: PlexClient, lib: PlexLibrary, rk: str)
     return col
 
 
+def _believed_children(plex: PlexClient, rk: str, meta: dict) -> list[dict] | None:
+    """A collection's members, or None when the answer is not believed: no members from a collection
+    whose own listing counts some (`childCount`) is a glitch, not a collection emptied — the same
+    empty-answer rule as the collection listing's (E32 review). Rebuilding on it showed the
+    collection empty until the next sync."""
+    children = plex.collection_children(rk)
+    count = str(meta.get("childCount") or "0")
+    if children or not count.isdigit() or int(count) == 0:
+        return children
+    log.warning(
+        "Plex listed no members for a non-empty collection; keeping its membership",
+        extra=logs.event("plex.sync.collection_children_empty", collection=rk),
+    )
+    return None
+
+
 def delete_collection_local(db: Session, lib: PlexLibrary, rk: str) -> None:
     """Local cleanup after a collection is deleted from Plex: strip rk from members + drop the row."""
     Model = PlexItem if lib.kind == "movie" else PlexShow
@@ -697,6 +718,19 @@ def delete_collection_local(db: Session, lib: PlexLibrary, rk: str) -> None:
     db.flush()
 
 
+def _rebuild_membership(
+    db: Session, lib: PlexLibrary, membership: dict[str, list[str]], kept: set[str]
+) -> None:
+    """Rebuild collection_keys on every member row of this library (clear + set), carrying over the
+    keys of a collection whose empty answer was not believed (`_believed_children`)."""
+    for Model in (PlexItem, PlexShow):
+        for row in db.execute(select(Model).where(Model.library_id == lib.id)).scalars():
+            held = [k for k in (row.collection_keys or []) if k in kept]
+            keys = [*membership.get(row.rating_key, []), *held]
+            row.collection_keys = list(dict.fromkeys(keys)) or None
+    db.flush()
+
+
 def _sync_collections(db: Session, plex: PlexClient, lib: PlexLibrary, stats: dict) -> None:
     """Mirror a library's collections + membership. Reads never touch Plex afterwards. Membership is
     written as `collection_keys` on member movies (plex_items) / shows (plex_shows). A full reconcile:
@@ -718,6 +752,7 @@ def _sync_collections(db: Session, plex: PlexClient, lib: PlexLibrary, stats: di
         )
         return
     seen: set[str] = set()
+    kept: set[str] = set()  # collections whose empty children answer is not believed
     membership: dict[str, list[str]] = {}  # member rating_key -> [collection rating_key, ...]
     for meta in cols:
         rk = str(meta.get("ratingKey") or "")
@@ -725,7 +760,10 @@ def _sync_collections(db: Session, plex: PlexClient, lib: PlexLibrary, stats: di
             continue
         _upsert_collection(db, existing.get(rk), rk, lib.id, meta)
         seen.add(rk)
-        for child in plex.collection_children(rk):
+        children = _believed_children(plex, rk, meta)
+        if children is None:
+            kept.add(rk)
+        for child in children or []:
             crk = str(child.get("ratingKey") or "")
             if crk:
                 membership.setdefault(crk, []).append(rk)
@@ -734,9 +772,4 @@ def _sync_collections(db: Session, plex: PlexClient, lib: PlexLibrary, stats: di
     for rk, col in existing.items():
         if rk not in seen:
             db.delete(col)
-    # Rebuild collection_keys on every member row of this library (clear + set).
-    for row in db.execute(select(PlexItem).where(PlexItem.library_id == lib.id)).scalars():
-        row.collection_keys = membership.get(row.rating_key) or None
-    for row in db.execute(select(PlexShow).where(PlexShow.library_id == lib.id)).scalars():
-        row.collection_keys = membership.get(row.rating_key) or None
-    db.flush()
+    _rebuild_membership(db, lib, membership, kept)