fix: Docker healthcheck connection refused

- Bind to 0.0.0.0 inside containers (DOCKER=true env marker)
- Healthcheck was failing because localhost resolved to IPv6 ::1
  but server was bound to IPv4 127.0.0.1
- External exposure still protected via Coolify port mapping to 127.0.0.1

Dockerfile

@@ -29,6 +29,9 @@ RUN npm run build
 # Ensure docs directory exists for /ui static files
 RUN mkdir -p docs
 
+# Docker environment marker (tells server.ts to bind to 0.0.0.0)
+ENV DOCKER=true
+
 # Expose port
 EXPOSE 3333