fix: Replace NATS healthcheck — ldm signal was triggering shutdown

The old healthcheck used nats-server --signal ldm which puts NATS into
lame duck (shutdown) mode. Use the /healthz HTTP endpoint instead.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -29,10 +29,10 @@ services:
     ports:
       - "8089:4222"   # Client connections
     healthcheck:
-      test: ["CMD", "nats-server", "--signal", "ldm"]
+      test: ["CMD-SHELL", "wget -qO- http://localhost:8222/healthz || exit 1"]
-      interval: 10s
+      interval: 5s
       timeout: 5s
-      retries: 3
+      retries: 5
 
   api:
     build: