Use turnutils_stunclient for coturn healthcheck

The coturn/coturn Alpine image doesn't include ss or netstat.
Use turnutils_stunclient (included in the coturn image) which
actually verifies the STUN server is responding, not just that
a port is open. Also add start_period to avoid false failures
during startup.

https://claude.ai/code/session_01Vm6NLaQuzM4VosAotqS1q8

docker-compose.yml

@@ -101,10 +101,11 @@ services:
         exec turnserver $$ARGS
     restart: unless-stopped
     healthcheck:
-      test: ["CMD-SHELL", "netstat -uln | grep -q ':3478'"]
-      interval: 30s
+      test: ["CMD", "turnutils_stunclient", "127.0.0.1"]
+      interval: 10s
       timeout: 5s
       retries: 3
+      start_period: 10s
 
 volumes:
   asterisk-config: