feat(infra): replace traefik path routing with nginx reverse proxy

- Add nginx service with /api, /ws, and / location routing
- Move traefik labels to proxy service only (port 80)
- Remove traefik labels from backend and frontend services
- Backend routes to :3001 via nginx upstream
- WebSocket upgrade enabled for /ws path

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker/Dockerfile.nginx

@@ -0,0 +1,7 @@
+FROM nginx:alpine
+
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80
+
+CMD ["nginx", "-g", "daemon off;"]