fix: host nginx proxies /api to gateway to avoid web DNS 502

Public HTTPS API/AI/gateway go to 127.0.0.1:8180; web only serves static. Restores static gateway upstream inside the web container.

Co-authored-by: Cursor <cursoragent@cursor.com>

web/nginx.conf

@@ -1,5 +1,4 @@
-# Dockerfile / 非 compose 场景：静态 upstream（启动时须能解析 gateway）。
-# compose 生产请用 nginx.conf.template + NGINX_ENTRYPOINT_LOCAL_RESOLVERS。
+# Dockerfile / 非 compose：静态 upstream。
 server {
     listen 80;
     server_name _;