fix: use dynamic upstream resolver in nginx api proxy

Add an explicit Docker resolver and switch the /api/ proxy to a
variable-based upstream so nginx re-resolves cold-display-guard-api
instead of caching stale DNS after the backend container restarts.

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

web/nginx.conf

@@ -2,11 +2,14 @@ server {
     listen 80;
     server_name _;
 
+    resolver 127.0.0.11 ipv6=off valid=10s;
+
     root /usr/share/nginx/html;
     index index.html;
 
     location /api/ {
-        proxy_pass http://cold-display-guard-api:19080;
+        set $api_upstream http://cold-display-guard-api:19080;
+        proxy_pass $api_upstream;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;