Add monthly offsite restore tests and automatic cleanup

systemd/app-restore-test.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Test the latest offsite backup on the first day of each month
+
+[Timer]
+OnCalendar=*-*-01 06:00:00
+RandomizedDelaySec=15m
+Persistent=true
+
+[Install]
+WantedBy=timers.target