Fix: use Docker internal DNS resolver for API upstream

nginx resolves proxy_pass hostnames at startup, before the api
container is reachable. Using resolver 127.0.0.11 + set $api
defers resolution to request time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/nginx.conf

@@ -9,8 +9,11 @@ server {
     }
 
     # Proxy API calls to the backend container
+    # Use Docker's internal DNS resolver so upstream resolves at request time
+    resolver 127.0.0.11 valid=10s;
+    set $api http://api:3001;
     location /api/ {
-        proxy_pass         http://api:3001/;
+        proxy_pass         $api/;
         proxy_set_header   Host              $host;
         proxy_set_header   X-Real-IP         $remote_addr;
         proxy_set_header   X-Forwarded-For   $proxy_add_x_forwarded_for;