feat(conf): the panel's backups are copied off site every day

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).

conf/systemd/webvirt-offsite.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Daily off-site copy of the WebVirtCloud panel backups
+
+[Timer]
+# after the panel's backup at 03:15
+OnCalendar=*-*-* 03:45
+Persistent=true
+
+[Install]
+WantedBy=timers.target