livekit-turn-setup.service detects the primary interface from the IPv4
default route, but had no ordering against network-online.target. With
NetworkManager+DHCP the default route is applied late at boot, so the
oneshot could run before it existed, exit 1, and — being a hard
dependency of livekit.service — take livekit down with it. The Hub then
showed a 'failed' red dot until livekit was restarted manually.
Order both livekit.service and livekit-turn-setup.service after
network-online.target. Also add a bounded retry when copying Caddy's ACME
cert so we never write an empty turn.crt/turn.key on a fresh boot.