fix(sync): an empty answer no longer empties a mirror; maintenance checks each video once a run

The catalog prune and the subscription import have long refused to reconcile against an empty
listing that contradicts what we hold (should_prune: a 200 with no items is a blip far more often
than a user who removed everything). Two mirrors did not: the YouTube playlist mirrors (dropped
every mirror) and the Plex collections (dropped every collection with its user-set `editable`
flag, and every member's keys — the guard landed in plex/sync.py with the previous commit). Both
now hold the same rule; test_empty_listing_guards covers each way (seen red).

sync/maintenance (19% covered) gets a lifecycle test: recover, grace, delete + one notification
per user, live stream kept, abandoned premiere. It found the rolling revalidation re-fetching the
same videos until `checked` reached the batch (15000 by default) whenever fewer videos were
available — quota spent on answers it already had; a run now skips what it has checked.

backend/tests/test_empty_listing_guards.py

@@ -0,0 +1,111 @@
+"""An empty answer is not an emptied library (E32 S2).
+
+Every mirror here reconciles: what the remote no longer lists is removed locally. The catalog
+(`_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.
+"""
+
+from sqlalchemy import select
+
+from app.models import Playlist, PlexCollection, PlexItem, PlexLibrary
+from app.plex import sync as plex_sync
+from app.sync import playlists as plsync
+
+
+class _Plex:
+    def __init__(self, collections: list[dict], children: dict[str, list[dict]]):
+        self._collections = collections
+        self._children = children
+
+    def collections(self, section_key):
+        return self._collections
+
+    def collection_children(self, rk):
+        return self._children.get(rk, [])
+
+
+def _library_with_a_collection(db):
+    lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
+    db.add(lib)
+    db.flush()
+    db.add(PlexCollection(rating_key="c1", library_id=lib.id, title="Bond", editable=True))
+    movie = PlexItem(
+        rating_key="m1", library_id=lib.id, kind="movie", title="Dr. No", collection_keys=["c1"]
+    )
+    db.add(movie)
+    db.commit()
+    return lib, movie
+
+
+def test_an_empty_collection_listing_keeps_the_collections_and_their_members(db):
+    lib, movie = _library_with_a_collection(db)
+    stats = {"collections": 0}
+    plex_sync._sync_collections(db, _Plex([], {}), lib, stats)
+    db.commit()
+    db.expire_all()
+    col = db.execute(select(PlexCollection).where(PlexCollection.rating_key == "c1")).scalar_one()
+    assert col.editable is True
+    assert db.get(PlexItem, movie.id).collection_keys == ["c1"]
+
+
+def test_a_listing_with_something_in_it_still_prunes_what_it_no_longer_has(db):
+    lib, movie = _library_with_a_collection(db)
+    stats = {"collections": 0}
+    other = {"ratingKey": "c2", "title": "Alien"}
+    plex_sync._sync_collections(db, _Plex([other], {"c2": []}), lib, stats)
+    db.commit()
+    db.expire_all()
+    keys = set(db.execute(select(PlexCollection.rating_key)).scalars())
+    assert keys == {"c2"}
+    assert db.get(PlexItem, movie.id).collection_keys is None
+
+
+class _YT:
+    def __init__(self, playlists: list[dict]):
+        self._playlists = playlists
+
+    def __call__(self, *_):
+        return self
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *_):
+        return False
+
+    def iter_my_playlists(self):
+        return self._playlists
+
+    def iter_playlist_items_with_ids(self, _ytid):
+        return []
+
+    def iter_my_playlist_video_ids(self, _ytid):
+        return []
+
+
+def test_an_empty_playlist_answer_keeps_the_youtube_mirrors(db, make_user, monkeypatch):
+    user = make_user()
+    db.add(Playlist(user_id=user.id, name="Mixes", source="youtube", yt_playlist_id="PL1"))
+    db.commit()
+    monkeypatch.setattr(plsync, "YouTubeClient", _YT([]))
+    plsync.pull_user_playlists(db, user)
+    db.expire_all()
+    names = db.execute(select(Playlist.name).where(Playlist.user_id == user.id)).scalars().all()
+    assert "Mixes" in names
+
+
+def test_a_playlist_answer_with_something_in_it_still_drops_a_vanished_mirror(
+    db, make_user, monkeypatch
+):
+    user = make_user()
+    db.add(Playlist(user_id=user.id, name="Mixes", source="youtube", yt_playlist_id="PL1"))
+    db.commit()
+    monkeypatch.setattr(plsync, "YouTubeClient", _YT([{"id": "PL2", "title": "New"}]))
+    plsync.pull_user_playlists(db, user)
+    db.expire_all()
+    ytids = set(
+        db.execute(select(Playlist.yt_playlist_id).where(Playlist.source == "youtube")).scalars()
+    )
+    assert "PL1" not in ytids