feat(infra) : add maintenance mode during deployments

Nginx returns a 503 page when maintenance.on exists. The deploy script
automatically enables/disables maintenance mode around the update.

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

infra/prod/nginx.conf

@@ -2,6 +2,23 @@ server {
     listen 80;
     server_name _;
 
+    # Maintenance mode
+    if (-f /var/www/html/maintenance.on) {
+        return 503;
+    }
+
+    error_page 503 @maintenance;
+
+    location @maintenance {
+        root /var/www/html/public;
+        rewrite ^(.*)$ /maintenance.html break;
+    }
+
+    location = /maintenance.html {
+        root /var/www/html/public;
+        internal;
+    }
+
     root /var/www/html/frontend/.output/public;
     index index.html;