fix(infra) : update proxy port to 8086 and add maintenance mode

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

infra/prod/nginx.conf

@@ -10,6 +10,16 @@ server {
     access_log /dev/stdout;
     error_log /dev/stderr;
 
+    # Mode maintenance : créer /var/www/html/maintenance.on pour activer
+    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 ^~ /api/ {
         root /var/www/html/public;
         try_files $uri /index.php?$query_string;