Function storage_save_series_audio_preference 

pub async fn storage_save_series_audio_preference(
    db: State<'_, DatabaseWrapper>,
    user_id: String,
    series_id: String,
    server_id: String,
    audio_track_display_title: Option<String>,
    audio_track_language: Option<String>,
    audio_track_index: Option<i32>,
) -> Result<(), String>

Save user’s preferred audio track for a series