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/bootstrap.php

@@ -232,6 +232,9 @@ require_once AKSRT_DIR . '/inc/classes/class-aksrt-download.php';
 // 付费内容收银台（post/_aksrt_pay → resource 订单）
 require_once AKSRT_DIR . '/inc/classes/class-aksrt-content-pay.php';
 
+// 论坛主题收银台（visibility=pay → resource 订单 + 多网关）
+require_once AKSRT_DIR . '/inc/classes/class-aksrt-topic-pay.php';
+
 // VIP 开通服务（package → ORDER_PRODUCT_VIP）
 require_once AKSRT_DIR . '/inc/classes/class-aksrt-vip-service.php';