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/Content/Download.php

@@ -118,7 +118,7 @@ class Download {
             : ((($meta['pay_modo'] ?? '0') === 'points') ? 'points' : '0');
         $display_price = is_array($priced) ? (float) ($priced['amount'] ?? 0) : (float) ($meta['price'] ?? 0);
 
-        $access = class_exists('Visibility')
+        $access = class_exists('AKSRT\\Content\\Visibility')
             ? Visibility::get_post_access_state($post, $uid)
             : [];