The daily backups stayed on the panel's host, so losing the host lost
them too. webvirt-offsite.timer now copies them at 03:45 with restic
over SFTP to another server, encrypted with a password only the
administrator and the host hold, and keeps 7 daily, 4 weekly and 6
monthly copies. It runs on the host as root, with Docker too, outside
the panel and its image. It fails when the newest backup is older
than 26 hours, so a stopped panel backup shows; an optional heartbeat
URL is called after each good copy.
The README has the setup, how to get a backup back from the copies,
and a fix found by the drill: moving to a new machine stopped when
the backup had no SSH keys (a panel without SSH computes).
