fix: a first watch state and a first Watch later survive two requests at once

The first checkpoint of an item and the final flush beside it both read no
state and both inserted; the loser failed on uq_plex_user_item. Two first saves
each made a Watch later list. Both are created by an ON CONFLICT DO NOTHING
insert and then read; a partial unique index holds one Watch later per user, and
the migration folds an existing second list into the first. Plex keeps one
per-item state map (rollup.item_state_map).

backend/tests/test_reader_service.py

@@ -308,3 +308,21 @@ def test_a_pin_is_one_per_thing_and_only_its_owners(db, make_user):
     pins.remove(db, pins.own_pin(db, user, pin.id))
     db.commit()
     assert db.execute(select(ComfortPin)).first() is None
+
+
+def test_a_watch_later_save_that_lost_the_race_uses_the_winners_list(db, make_user, monkeypatch):
+    """The first two saves at once both read "no Watch later"; the second must land in the list the
+    first made, not make one of its own."""
+    user = make_user()
+    won = service.watch_later(db, user)
+    db.commit()
+    real = queries.watch_later
+    stale = [True]  # this request's first read happened before the winner committed
+
+    def read(d, u):
+        return None if stale and stale.pop() else real(d, u)
+
+    monkeypatch.setattr(queries, "watch_later", read)
+    assert service.watch_later(db, user).id == won.id
+    db.commit()
+    assert [p.id for p in queries.playlists_for(db, user) if p.kind == "watch_later"] == [won.id]