fix(youtube): "this playlist no longer exists" only where the user picked that list

playlistNotFound was mapped to "gone" everywhere: exploring a channel without uploads (its
uploads list answers not found) and a list deleted mid bulk pull both said a playlist no
longer exists. The reason is now "gone" only on single-playlist routes; explore reads it
as an empty back-catalogue, and the bulk pull skips that list and syncs the rest.

backend/tests/test_empty_listing_guards.py

@@ -163,6 +163,33 @@ def test_a_playlist_answer_with_something_in_it_still_drops_a_vanished_mirror(
     assert "PL1" not in ytids
 
 
+class _YTOneGone(_YT):
+    """A list deleted on YouTube between the listing and the read of its items."""
+
+    def iter_my_playlist_video_ids(self, ytid):
+        if ytid == "PLgone":
+            raise YouTubeError("m", reason="playlistNotFound", path="playlistItems")
+        return []
+
+
+def test_a_list_deleted_mid_pull_is_skipped_and_the_rest_still_sync(db, make_user, monkeypatch):
+    # Not "this playlist no longer exists" for a bulk pull the user never named a list in, and not
+    # a failure for every other list either (E32 review round 2): the next pull drops the mirror.
+    user = make_user()
+    db.commit()
+    listing = [
+        {"id": "PLgone", "title": "Gone", "item_count": 3},
+        {"id": "PL2", "title": "Kept", "item_count": 0},
+    ]
+    monkeypatch.setattr(plsync, "YouTubeClient", _YTOneGone(listing))
+    assert plsync.pull_user_playlists(db, user) == {"synced": 1}
+    db.expire_all()
+    ytids = set(
+        db.execute(select(Playlist.yt_playlist_id).where(Playlist.user_id == user.id)).scalars()
+    )
+    assert ytids == {"PL2"}
+
+
 def _mirror_with_an_item(db, user, **kw):
     db.add(Channel(id="UC_pl", title="C"))
     db.flush()