Add nginx HTTP Basic Auth for production

A custom entrypoint generates /etc/nginx/.htpasswd at container startup
from AUTH_USER and AUTH_PASSWORD env vars (via openssl). No credentials
are baked into the image. Pass AUTH_USER and AUTH_PASSWORD in docker-compose.

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

nginx/default.conf.template

@@ -3,6 +3,9 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
+    auth_basic "Football App";
+    auth_basic_user_file /etc/nginx/.htpasswd;
+
     # Proxy /api/* → football-data.org, injecting the API key server-side
     location /api/ {
         proxy_pass         https://api.football-data.org/v4/;