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/retrieve_smtp.php

@@ -3,13 +3,17 @@
     require_once '../../../assets/utils/db_auth.php';
     require_once '../../../assets/utils/classes/SmtpManager.php';
 
+    // Admin data: owners and admins only; staff and viewers never see it.
+    require_role($user_role, ['owner', 'admin']);
+
     try {
         $smtp = new SmtpManager($pdo1, $company_id, $method, $pinkey, $iv);
         $answer['success'] = 1;
         $answer['output']  = $smtp->get();
 
     } catch (Exception $e) {
-        $answer['message'] = $e->getMessage();
+        error_log('[retrieve_smtp] ' . $e->getMessage());
+        $answer['message'] = 'Could not load SMTP settings.';
         http_response_code(500);
     }