unattended-upgrades was already active on the host, just never documented
or closed. Adds a generic, reusable systemd timer + script that fires
before the daily update window and notifies via email and a Matrix thread
reply if any packages are actually pending - reusing the mas-cli bot
account pattern established for Draupnir.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
