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_empty_listing_guards.py

@@ -4,14 +4,26 @@ Every mirror here reconciles: what the remote no longer lists is removed locally
 (`_prune_stale`) and the subscriptions (`should_prune`) had long refused to do that on an empty
 listing that contradicts what we hold — a 200 with no items is a blip far more often than a user
 who removed everything. Two mirrors did not: the Plex collections (a row carries the user-set
-`editable` flag, and every member its keys) and the YouTube playlist mirrors. These hold both.
+`editable` flag, and every member its keys) and the YouTube playlist mirrors. These hold both — and,
+one level down (E32 review), a collection's members and a playlist's items: an empty answer that
+contradicts the remote's own count is not believed either.
 """
 
+import pytest
 from sqlalchemy import select
 
-from app.models import Playlist, PlexCollection, PlexItem, PlexLibrary
+from app.models import (
+    Channel,
+    Playlist,
+    PlaylistItem,
+    PlexCollection,
+    PlexItem,
+    PlexLibrary,
+    Video,
+)
 from app.plex import sync as plex_sync
 from app.sync import playlists as plsync
+from app.youtube.client import YouTubeError
 
 
 class _Plex:
@@ -25,6 +37,9 @@ class _Plex:
     def collection_children(self, rk):
         return self._children.get(rk, [])
 
+    def metadata(self, rk):
+        return next((c for c in self._collections if c.get("ratingKey") == rk), {})
+
 
 def _library_with_a_collection(db):
     lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
@@ -62,9 +77,40 @@ def test_a_listing_with_something_in_it_still_prunes_what_it_no_longer_has(db):
     assert db.get(PlexItem, movie.id).collection_keys is None
 
 
+def test_no_members_from_a_collection_plex_counts_as_non_empty_keeps_its_members(db):
+    lib, movie = _library_with_a_collection(db)
+    stats = {"collections": 0}
+    bond = {"ratingKey": "c1", "title": "Bond", "childCount": 2}
+    plex_sync._sync_collections(db, _Plex([bond], {"c1": []}), lib, stats)
+    db.commit()
+    db.expire_all()
+    assert db.get(PlexItem, movie.id).collection_keys == ["c1"]
+
+
+def test_a_collection_plex_counts_as_empty_does_lose_its_members(db):
+    lib, movie = _library_with_a_collection(db)
+    stats = {"collections": 0}
+    bond = {"ratingKey": "c1", "title": "Bond", "childCount": 0}
+    plex_sync._sync_collections(db, _Plex([bond], {"c1": []}), lib, stats)
+    db.commit()
+    db.expire_all()
+    assert db.get(PlexItem, movie.id).collection_keys is None
+
+
+@pytest.mark.parametrize(("count", "kept"), [(2, ["c1"]), (0, None)])
+def test_a_collection_resync_believes_an_empty_member_list_only_with_a_zero_count(db, count, kept):
+    lib, movie = _library_with_a_collection(db)
+    bond = {"ratingKey": "c1", "title": "Bond", "childCount": count}
+    plex_sync.resync_collection(db, _Plex([bond], {"c1": []}), lib, "c1")
+    db.commit()
+    db.expire_all()
+    assert db.get(PlexItem, movie.id).collection_keys == kept
+
+
 class _YT:
-    def __init__(self, playlists: list[dict]):
+    def __init__(self, playlists: list[dict], snippet: dict | None = None):
         self._playlists = playlists
+        self._snippet = snippet
 
     def __call__(self, *_):
         return self
@@ -84,6 +130,12 @@ class _YT:
     def iter_my_playlist_video_ids(self, _ytid):
         return []
 
+    def get_playlist_snippet(self, _ytid):
+        return self._snippet
+
+    def get_videos(self, _ids):
+        return []
+
 
 def test_an_empty_playlist_answer_keeps_the_youtube_mirrors(db, make_user, monkeypatch):
     user = make_user()
@@ -109,3 +161,72 @@ def test_a_playlist_answer_with_something_in_it_still_drops_a_vanished_mirror(
         db.execute(select(Playlist.yt_playlist_id).where(Playlist.source == "youtube")).scalars()
     )
     assert "PL1" not in ytids
+
+
+def _mirror_with_an_item(db, user, **kw):
+    db.add(Channel(id="UC_pl", title="C"))
+    db.flush()
+    db.add(Video(id="v1", channel_id="UC_pl", title="One"))
+    pl = Playlist(user_id=user.id, name="Mixes", yt_playlist_id="PL1", **kw)
+    db.add(pl)
+    db.flush()
+    db.add(PlaylistItem(playlist_id=pl.id, video_id="v1", position=0))
+    db.commit()
+    return pl
+
+
+def _items(db, pl):
+    db.expire_all()
+    return (
+        db.execute(select(PlaylistItem.video_id).where(PlaylistItem.playlist_id == pl.id))
+        .scalars()
+        .all()
+    )
+
+
+def test_no_items_from_a_playlist_youtube_counts_as_non_empty_keeps_the_mirror_items(
+    db, make_user, monkeypatch
+):
+    user = make_user()
+    pl = _mirror_with_an_item(db, user, source="youtube")
+    listing = [{"id": "PL1", "title": "Mixes", "item_count": 1}]
+    monkeypatch.setattr(plsync, "YouTubeClient", _YT(listing))
+    plsync.pull_user_playlists(db, user)
+    assert _items(db, pl) == ["v1"]
+
+
+def test_a_playlist_youtube_counts_as_empty_is_emptied(db, make_user, monkeypatch):
+    user = make_user()
+    pl = _mirror_with_an_item(db, user, source="youtube")
+    listing = [{"id": "PL1", "title": "Mixes", "item_count": 0}]
+    monkeypatch.setattr(plsync, "YouTubeClient", _YT(listing))
+    plsync.pull_user_playlists(db, user)
+    assert _items(db, pl) == []
+
+
+def test_a_reset_to_youtube_refuses_a_blank_listing_and_keeps_the_items(db, make_user, monkeypatch):
+    user = make_user()
+    pl = _mirror_with_an_item(db, user, source="youtube")
+    monkeypatch.setattr(plsync, "YouTubeClient", _YT([], {"title": "Mixes", "item_count": 1}))
+    with pytest.raises(YouTubeError) as err:
+        plsync.repull_playlist(db, user, pl)
+    db.rollback()
+    assert err.value.reason == "emptyAnswer"
+    assert _items(db, pl) == ["v1"]
+
+
+def test_a_reset_of_a_deleted_playlist_says_it_is_gone(db, make_user, monkeypatch):
+    user = make_user()
+    pl = _mirror_with_an_item(db, user, source="youtube")
+    monkeypatch.setattr(plsync, "YouTubeClient", _YT([], None))
+    with pytest.raises(YouTubeError) as err:
+        plsync.repull_playlist(db, user, pl)
+    assert err.value.reason == "playlistNotFound"
+
+
+def test_a_push_plan_refuses_a_blank_listing_rather_than_reading_it_as_all_deleted(db, make_user):
+    user = make_user()
+    pl = _mirror_with_an_item(db, user, source="local")
+    with pytest.raises(YouTubeError) as err:
+        plsync.plan_merge(db, user, pl, live=([], {"title": "Mixes", "item_count": 1}))
+    assert err.value.reason == "emptyAnswer"

backend/tests/test_playlist_merge.py

@@ -12,8 +12,8 @@ import pytest
 
 from app.models import Channel, Playlist, PlaylistItem, Video
 from app.sync import playlists as pl_mod
+from app.sync.playlist_merge import merge_plan
 from app.sync.playlists import (
-    merge_plan,
     merge_playlist,
     parse_baseline,
     parse_unavailable,

backend/tests/test_plex_client.py

@@ -81,6 +81,9 @@ def test_a_section_page_asks_for_guids_and_reports_the_server_total(plex):
 
     plex.routes["/library/sections/7/all"] = {"Metadata": [], "size": 0}
     assert plex.section_items("7", 2) == ([], 0)
+    # No total at all is not "0": the caller must not read it as a complete, empty section.
+    plex.routes["/library/sections/7/all"] = {"Metadata": []}
+    assert plex.section_items("7", 2) == ([], None)
 
 
 def test_related_flattens_the_hubs_and_degrades_to_empty(plex):

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)

backend/tests/test_source_size.py

@@ -35,7 +35,6 @@ BIG_MODULES: dict[str, int] = {
     "app/routes/admin.py": 864,
     "app/routes/auth.py": 1353,
     "app/routes/downloads.py": 1237,
-    "app/sync/playlists.py": 843,
 }
 
 COMPLEX: dict[str, list[int]] = {

backend/tests/test_youtube_refusals.py

@@ -27,6 +27,9 @@ from app.youtube.client import YouTubeError, refusal_reason
         (YouTubeError("No refresh token; user must re-authenticate"), "reconnect"),
         (YouTubeError("m", reason="transport"), "unavailable"),
         (YouTubeError("m", reason="backendError", path="subscriptions"), "unavailable"),
+        # A playlist deleted on YouTube: neither a retry nor a reconnect helps (it said "reconnect").
+        (YouTubeError("m", reason="playlistNotFound", path="playlists"), "gone"),
+        (YouTubeError("m", reason="emptyAnswer", path="playlistItems"), "unavailable"),
     ],
 )
 def test_a_failure_is_sorted_by_what_the_user_can_do(exc, reason):