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/conf.d/00-upstreams.conf

@@ -0,0 +1,30 @@
+# ============================================================
+# 上游定义 - 所有反向代理目标集中在此
+# ============================================================
+
+# --- Coze 业务后端 (coze-network) ---
+upstream coze_backend {
+    server coze-server:8888;
+    keepalive 16;
+}
+
+upstream coze_minio {
+    server coze-minio:9000;
+    keepalive 16;
+}
+
+# --- Kong AI Gateway 相关 (kong-ai-gateway-prod_kong-net) ---
+upstream kong_proxy {
+    server kong:8000;
+    keepalive 32;
+}
+
+upstream admin_portal {
+    server admin-portal:8080;
+    keepalive 4;
+}
+
+upstream grafana_ui {
+    server grafana:3000;
+    keepalive 4;
+}