Add optional tid_v2 across Studio, config/DB, CLI, Web UI, stream-gears,
and Python uploaders so new Bilibili partition IDs can be set alongside
legacy tid. Fixes#1662; related to #1562.
Refactored the DownloadTask::execute method to return DownloadStatus and adjusted its visibility. Enhanced DActor message handling to update stream info after download execution. Updated README and introduction guide with improved instructions for background running and systemd service configuration.