* fix: a restarted server is ready only when its own tunnel is

cloudflared kept serving /ready for its default 30 s grace period after SIGTERM, so a restart's readiness check could hit the dying process and return early while links still gave 530. cloudflared now gets --grace-period 2s, and the serve exit trap waits for its children before the pidfile goes.

* test: print the server and caddy logs when a check fails