fix(ns): class_exists() must use FQCN after PSR-4

Bare class_exists('Balance') looks up global Balance, always false
under AKSRT\* namespaces → REST 403 on balance exchange and friends.
Rewrite string checks to AKSRT\{Domain}\{Class}.

src/AKSRT/Auth/EmailRollback.php

@@ -167,7 +167,7 @@ class EmailRollback {
         if (count($log) > 10) $log = array_slice($log, -10);
         update_user_meta($user_id, 'aksrt_email_verification_admin_log', $log);
 
-        if (class_exists('Service')) {
+        if (class_exists('AKSRT\\Auth\\Service')) {
             Service::log_event('admin.email_rollback', [
                 'uid' => $user_id,
                 'by'  => $by_admin_id ?: get_current_user_id(),