Enforce roles on admin endpoints and return real status codes

- users, SMTP and batch-lock endpoints are owner/admin only
- engines answer 400/403/404/409/500 instead of 200 with an error body;
  database errors no longer leak to the client

app/setting/api/engine/gl_maintenance.php

@@ -34,6 +34,7 @@ try {
     if ($action === 'rebuild_period') {
         $period = $data['period'] ?? '';
         if (!preg_match('/^\d{4}-\d{2}$/', $period)) {
+            http_response_code(400);
             $answer['message'] = 'Invalid period.';
             exit(json_encode($answer));
         }