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/scripts-traits/trait-aksrt-scripts-part-1b.php

@@ -661,8 +661,12 @@ trait AKSRT_Scripts_Part_1b {
             $enqueue_module('resource-archive');
         }
 
-        // 单篇资源 / 付费内容 / 独立下载页：下载 + 收银台脚本
-        if ($is_single_resource || $is_paid_content_single || $is_download_file_page) {
+        // 付费论坛主题：多网关收银台（与内容共用 cms-download.js）
+        $is_pay_forum_topic = is_singular('aksrt_forum_topic')
+            && sanitize_key((string) get_post_meta(get_queried_object_id(), '_aksrt_visibility', true)) === 'pay';
+
+        // 单篇资源 / 付费内容 / 独立下载页 / 付费主题：下载 + 收银台脚本
+        if ($is_single_resource || $is_paid_content_single || $is_download_file_page || $is_pay_forum_topic) {
             $cms_download_js_path = get_template_directory() . '/assets/js/cms-download.js';
             wp_enqueue_script(
                 'aksrt-cms-download',