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/nginx.conf

@@ -0,0 +1,14 @@
+worker_processes 1;
+events { worker_connections 1024; }
+
+http {
+    include       mime.types;
+    default_type  application/octet-stream;
+    sendfile        on;
+    keepalive_timeout  65;
+
+    # setting client request body size limit - 0 means no limit
+    client_max_body_size 0;
+
+    include /etc/nginx/conf.d/*.conf;
+}