fix(deploy): preserve /api prefix in nginx proxy_pass

deploy/nginx/docker-chat.conf

@@ -6,7 +6,8 @@ server {
   index index.html;
 
   location /api/ {
-    proxy_pass http://backend:4000/;
+    # preserve the original URI (/api/...) when proxying
+    proxy_pass http://backend:4000;
     proxy_http_version 1.1;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection 'upgrade';