Function download_item_and_start 

pub async fn download_item_and_start(
    db: State<'_, DatabaseWrapper>,
    smart_cache: State<'_, SmartCacheWrapper>,
    download_manager: State<'_, DownloadManagerWrapper>,
    app: AppHandle,
    request: DownloadItemAndStartRequest,
) -> Result<i64, String>

Queue and start a download in a single atomic operation This simplifies the frontend flow by combining multiple steps