fix: redirect unprefixed paths to /en/ default language

/about/ and /tags/ were 404ing after Docker migration since the
redirect rule was previously in the gateway nginx config.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nginx.conf

@@ -27,6 +27,11 @@ server {
         add_header Cache-Control "public, must-revalidate";
     }
 
+    # Redirect unprefixed paths to /en/
+    location ~ ^/(?!en/|de/|css/|js/|img/|favicon|android|apple|site|llms) {
+        return 302 /en$request_uri;
+    }
+
     # Clean URLs
     location / {
         try_files $uri $uri/ $uri.html =404;