Debug Watch List Movie Operations

You are given a full-stack Movie DB application. Users can log in, create, update, and delete watch lists, and add or remove movies from a watch list. Several unit tests are failing around watch-list movie operations. Debug and fix the backend logic so the following scenarios work correctly: 1. Add a movie to an existing watch list. 2. Add a movie that is already present in the watch list. 3. Remove a movie from an existing watch list. 4. Add many movies to a watch list, then remove them one by one. 5. Try to add a movie to a watch list that does not exist. 6. Try to remove a movie from a watch list that does not exist. Your fix should ensure that: - The code checks whether the watch list exists before modifying it. - The code checks whether the movie exists when adding a movie. - Duplicate movies are not added to the same watch list. - Removing a movie updates the persisted watch list correctly. - All database changes are saved before returning a response. - Each failure case returns the correct HTTP status code and error response. Assume a conventional REST contract such as: - `POST /watchlists/{watchListId}/movies` adds a movie. - `DELETE /watchlists/{watchListId}/movies/{movieId}` removes a movie. - `404 Not Found` is returned when the watch list or movie does not exist. - `409 Conflict` is returned when attempting to add a duplicate movie. - `201 Created` or `200 OK` is returned for a successful add. - `200 OK` or `204 No Content` is returned for a successful delete, depending on the existing API convention.