fix: preserve /api prefix in nginx reverse proxy

- Changed proxy_pass from http://backend/ to http://backend
- Trailing slash was stripping /api prefix causing 404 errors
- Backend expects /api/stats not /stats

Dockerfile.frontend

@@ -81,7 +81,7 @@ server {
         limit_req zone=api_limit burst=20 nodelay;
 
         # Proxy settings
-        proxy_pass http://backend/;
+        proxy_pass http://backend;
         proxy_http_version 1.1;
 
         # Headers