Add SABnzbd as Usenet download client alongside Transmission for torrents.
Radarr and Sonarr can now use both download methods.

- Add transmission-gc timer to remove completed torrents after ratio >= 5.0
or age >= 30 days, deleting files to free disk space
- Enable ratio-limit and seed-queue in transmission settings
- Fix transmission-watcher to check service state before restarting,
preventing restart loops during startup

The session keyring (@s) wasn't persisting keys when accessed from
systemd user services. Switch to user keyring (@u) which is shared
across all processes for the same UID.

Fix "too many open files" error by setting LimitNOFILE=65536