refactor: extract nginx to standalone nginx-gateway deployment

- Remove coze-web service from docker-compose.yml
- Replace nginx second stage in frontend/Dockerfile with alpine dist-only stage
- Add nginx-gateway/ with standalone nginx container deployment
  - docker-compose.yml joining both coze-network and kong-net
  - Split nginx configs into 6 per-domain files:
    - 00-upstreams.conf (shared upstreams)
    - 10-default-server.conf (catch-all 444)
    - 20-coze.conf (coze studio)
    - 30-kong-api.conf (kong ai gateway)
    - 40-admin-portal.conf (admin portal)
    - 50-grafana.conf (grafana dashboard)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nginx-gateway/docker-compose.yml

@@ -0,0 +1,27 @@
+name: nginx-gateway
+
+services:
+  nginx-gateway:
+    image: nginx:1.25-alpine
+    container_name: nginx-gateway
+    restart: always
+    ports:
+      - "80:80"
+      - "443:443"
+    volumes:
+      - ./nginx.conf:/etc/nginx/nginx.conf:ro
+      - ./conf.d:/etc/nginx/conf.d:ro
+      - ./ssl:/etc/nginx/ssl:ro
+      - ./dist:/usr/share/nginx/html:ro
+      - /etc/localtime:/etc/localtime:ro
+      - /etc/timezone:/etc/timezone:ro
+    networks:
+      - coze-network
+      - kong-net
+
+networks:
+  coze-network:
+    external: true
+  kong-net:
+    external:
+      name: kong-ai-gateway-prod_kong-net