Function repository_get_audio_only_stream_url_for_video 

pub async fn repository_get_audio_only_stream_url_for_video(
    manager: State<'_, RepositoryManagerWrapper>,
    handle: String,
    item_id: String,
    media_source_id: Option<String>,
    start_time_seconds: Option<f64>,
    audio_stream_index: Option<i32>,
) -> Result<String, String>

Get an audio-only stream URL for a video item (background-audio handoff).

TRACES: UR-040 | JA-032 | UT-061