fix(polish): 订单购物车论坛粉边软化 + 论坛付费帖购链路

- 购物车/商品/论坛表面复用 #77 soft recipe（mix 边 + soft ring）
- 论坛 visibility=pay：订单 resource + 余额扣费闭环
- VIP 价/免费解锁 + 购买 CTA/AJAX，复用现有 Order/Credits

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

inc/classes/class-aksrt-forum.php

@@ -144,6 +144,13 @@ class AKSRT_Forum {
         return ['enabled' => false];
     }
 
+    /**
+     * Alias kept for topic-service helpers that still call get_forum_config().
+     */
+    public function get_forum_config(): array {
+        return $this->get_config();
+    }
+
     public function get_boards(array $args = []): array {
         return $this->board_service()->get_boards($args);
     }
@@ -345,6 +352,7 @@ class AKSRT_Forum {
         add_action('wp_ajax_aksrt_forum_create_topic', [$this, 'ajax_create_topic']);
         add_action('wp_ajax_aksrt_forum_follow_plate', [$this, 'ajax_follow_plate']);
         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']);
@@ -446,6 +454,26 @@ 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.
+     *
+     * @return array{success:bool,message:string}
+     */
+    public function success(string $message = '', array $extra = []): array {
+        return array_merge(['success' => true, 'message' => $message], $extra);
+    }
+
+    /**
+     * @return array{success:bool,message:string}
+     */
+    public function error(string $message = '', array $extra = []): array {
+        return array_merge(['success' => false, 'message' => $message], $extra);
+    }
+
     public function apply_moderator(int $user_id, int $board_id, string $reason) {
         return $this->moderation_service()->apply_moderator($user_id, $board_id, $reason);
     }