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/tests/test_plex_watch_reconcile.py

@@ -13,11 +13,13 @@ from app.plex import watch_sync
 
 class _FakePlex:
     """A PlexClient stand-in: one movie section whose listing reports `total` (more than it hands
-    over = a walk cut short), and a record of every write the reconcile sends back to Plex."""
+    over = a walk cut short), plus any `more` sections as `(type, leaves, total)` — listed AFTER
+    the movies — and a record of every write the reconcile sends back to Plex."""
 
-    def __init__(self, leaves: list[dict], total: int | None = None):
-        self.leaves = leaves
-        self.total = len(leaves) if total is None else total
+    def __init__(self, leaves: list[dict], total: int | None = None, more=()):
+        self.listings = {"1": ("movie", leaves, len(leaves) if total is None else total)}
+        for i, (stype, extra, extra_total) in enumerate(more, start=2):
+            self.listings[str(i)] = (stype, extra, extra_total)
         self.calls: list[tuple] = []
 
     def __enter__(self):
@@ -27,10 +29,11 @@ class _FakePlex:
         return False
 
     def sections(self):
-        return [{"type": "movie", "key": "1"}]
+        return [{"type": stype, "key": key} for key, (stype, _l, _t) in self.listings.items()]
 
     def section_items(self, key, item_type, start, size):
-        return self.leaves[start : start + size], self.total
+        _stype, leaves, total = self.listings[key]
+        return leaves[start : start + size], total
 
     def scrobble(self, rk):
         self.calls.append(("scrobble", rk))
@@ -136,6 +139,45 @@ def test_a_scan_cut_short_clears_nothing(db, monkeypatch, world):
     assert out["cleared"] == 0 and out["incomplete"] == 1
 
 
+_LISTED = [
+    {"ratingKey": "m3"},
+    {"ratingKey": "m4", "viewCount": 1, "lastViewedAt": 1_700_000_000},
+    {"ratingKey": "m5", "viewOffset": 120_000, "lastViewedAt": 1_700_000_000},
+]
+
+
+def test_a_genuinely_empty_second_section_still_lets_the_reconcile_clear(db, monkeypatch, world):
+    # The server SAYS the shows section holds 0: that is a complete listing, not a hiccup — before
+    # the E32 review it read as "incomplete" forever and un-watches never synced again.
+    state, rows = world
+    _local_states(state)
+    db.commit()
+    out = _run(db, monkeypatch, _FakePlex(_LISTED, more=[("show", [], 0)]))
+    assert "m1" not in rows()
+    assert out["cleared"] == 1 and "incomplete" not in out
+
+
+def test_a_cut_short_section_is_not_rescued_by_a_complete_one_after_it(db, monkeypatch, world):
+    # Completeness is every section's, not the last one's: movies cut short (5 said, 3 handed),
+    # then a complete shows listing — m1 may simply not have been reached.
+    state, rows = world
+    _local_states(state)
+    db.commit()
+    shows = [{"ratingKey": "s1"}]
+    out = _run(db, monkeypatch, _FakePlex(_LISTED, total=5, more=[("show", shows, 1)]))
+    assert rows()["m1"] == ("watched", 0, True)
+    assert out["cleared"] == 0 and out["incomplete"] == 1
+
+
+def test_a_listing_without_a_total_is_never_complete(db, monkeypatch, world):
+    state, rows = world
+    _local_states(state)
+    db.commit()
+    out = _run(db, monkeypatch, _FakePlex(_LISTED, more=[("show", [], None)]))
+    assert rows()["m1"] == ("watched", 0, True)
+    assert out["cleared"] == 0 and out["incomplete"] == 1
+
+
 def test_an_empty_listing_is_not_a_library_of_unwatched_titles(db, monkeypatch, world):
     state, rows = world
     state("m1", status="watched", synced_to_plex=True)