Function spawn_download_worker 

fn spawn_download_worker(
    app: AppHandle,
    download_id: i64,
    item_id: String,
    stream_url: String,
    target_path: PathBuf,
    active_downloads: Arc<Mutex<HashSet<i64>>>,
)

Spawn the background worker for one download. On completion or failure it unregisters the slot, emits the terminal event, and pumps the queue so the next pending download starts automatically.