feat(forum): multi-gateway cashier for pay-visibility topics

Replace balance-only AJAX topic purchase with AKSRT_Topic_Pay REST
cashier (resource orders + Payment_Gateway), reuse cms-download modal,
and unlock on aksrt_order_paid.

inc/classes/class-aksrt-forum.php

@@ -353,7 +353,6 @@ class AKSRT_Forum {
         add_action('wp_ajax_aksrt_forum_follow_plate', [$this, 'ajax_follow_plate']);
         add_action('wp_ajax_aksrt_forum_purchase_plate_follow', [$this, 'ajax_purchase_plate_follow']);
         add_action('wp_ajax_aksrt_forum_unlock_topic', [$this, 'ajax_unlock_topic']);
-        add_action('wp_ajax_aksrt_forum_purchase_topic', [$this, 'ajax_purchase_topic']);
 
         // §7.4 draft persistence
         add_action('wp_ajax_aksrt_forum_save_draft', [$this, 'ajax_save_draft']);
@@ -459,10 +458,6 @@ class AKSRT_Forum {
         $this->topic_service()->ajax_unlock_topic();
     }
 
-    public function ajax_purchase_topic(): void {
-        $this->topic_service()->ajax_purchase_topic();
-    }
-
     /**
      * Lightweight result helper for topic service actions.
      *
@@ -607,6 +602,10 @@ class AKSRT_Forum {
         return $this->topic_service()->resolve_topic_pay_price($topic_id, $user_id);
     }
 
+    public function mark_topic_unlocked(int $user_id, int $topic_id): void {
+        $this->topic_service()->mark_topic_unlocked($user_id, $topic_id);
+    }
+
     public function prepare_vote_meta(array $data): array {
         return $this->topic_service()->prepare_vote_meta($data);
     }