fix: 前端自动化部署优化

nginx.conf

@@ -39,10 +39,9 @@ server {
         access_log off;
     }
 
-    # API proxy to backend (container network)
+    # API proxy to backend (configurable via $API_UPSTREAM)
     location /api/ {
-        # Keep original request path; backend service name resolves inside Docker network
-        proxy_pass http://backend:18888;
+        proxy_pass $API_UPSTREAM;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;