Let a server service carry helper scripts and timers, starting with UseSend's cert refresh

server/vps/UseSend/usesend-cert-refresh.timer

@@ -0,0 +1,12 @@
+[Unit]
+Description=Daily check for a renewed UseSend SMTP proxy certificate
+
+[Timer]
+OnCalendar=daily
+# Spread the load and avoid landing exactly when certbot is writing.
+RandomizedDelaySec=2h
+# Catch up after the host has been off; the cert does not care that we were asleep.
+Persistent=true
+
+[Install]
+WantedBy=timers.target