When integrating Apple Music via MusicKit JS, the music_user_token returned by authorize() is NOT stable across sessions. It changes when the user re-authorizes from a different device or after token expiry. Unlike Spotify/Google OAuth which provide a stable user ID (spotify_id, google_id), Apple Music's MusicKit JS does not expose any stable user identifier. Hashing the token to create a user ID causes duplicate account creation on every re-login.