caddy/coredns declare Requires=orama-node.service, so stopping orama-node
cascade-stops them; Requires propagates STOP but not START, and
StartServicesOrdered only starts the orama services — so a bare
'orama node restart' left caddy dead and the node's :443 HTTPS frontend
offline until the next reboot. Capture the active frontend units before the
stop and restart exactly those after the gateway is healthy. Adds
ServiceUnitExists helper + selectFrontendToRestore policy test.