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/Payment/CreatorRevenue.php

@@ -26,7 +26,7 @@ class CreatorRevenue {
          * Whether creator revenue settlement is enabled.
          */
         private function is_enabled(): bool {
-            if (!class_exists('Revenue') || !class_exists('\AKSRT\Settings\Settings')) {
+            if (!class_exists('AKSRT\\Payment\\Revenue') || !class_exists('\AKSRT\Settings\Settings')) {
                 return false;
             }
             $settings = \AKSRT\Settings\Settings::get_payment_revenue_settings();