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>

assets/js/forum-governance.js

@@ -675,6 +675,49 @@
         });
     }
 
+    function bindPurchaseButton(button) {
+        button.addEventListener('click', function () {
+            var topicId = parseInt(button.getAttribute('data-topic-id') || '0', 10);
+            var price = button.getAttribute('data-price') || '0';
+            var isFree = button.getAttribute('data-free') === '1';
+            var feedback = button.parentElement ? button.parentElement.querySelector('.js-aksrt-action-feedback') : null;
+
+            if (!topicId) {
+                return;
+            }
+
+            var confirmMsg = isFree
+                ? '确认使用 VIP 免费解锁此主题？'
+                : ('确认使用余额支付 ¥' + price + ' 购买此主题？');
+            if (!window.confirm(confirmMsg)) {
+                return;
+            }
+
+            button.disabled = true;
+            setFeedback(feedback, isFree ? '正在解锁…' : '正在支付…', 'is-loading');
+
+            var data = new FormData();
+            data.append('action', 'aksrt_forum_purchase_topic');
+            data.append('topic_id', topicId);
+            data.append('_nonce', rootData.forumNonce || '');
+
+            fetch((rootData.ajaxUrl || '/wp-admin/admin-ajax.php'), {
+                method: 'POST',
+                body: data,
+                credentials: 'same-origin',
+            }).then(function (r) { return r.json(); }).then(function (payload) {
+                if (!payload.success) {
+                    throw new Error((payload.data && payload.data.message) || '购买失败');
+                }
+                setFeedback(feedback, '购买成功，正在刷新…', 'is-success');
+                setTimeout(function () { window.location.reload(); }, 800);
+            }).catch(function (error) {
+                setFeedback(feedback, error.message || '购买失败', 'is-error');
+                button.disabled = false;
+            });
+        });
+    }
+
     function bindComposeCoverToggle(form) {
         var coverTypeSelect = form.querySelector('#compose-cover-type');
         var imageField = form.querySelector('#compose-cover-image-field');
@@ -1411,6 +1454,7 @@
         document.querySelectorAll('.js-aksrt-topic-score').forEach(bindScoreButton);
         document.querySelectorAll('.js-aksrt-adopt-answer').forEach(bindAdoptButton);
         document.querySelectorAll('.js-aksrt-unlock-topic').forEach(bindUnlockButton);
+        document.querySelectorAll('.js-aksrt-purchase-topic').forEach(bindPurchaseButton);
         bindForumCommentEngine();
         bindForumHomeTabs();
         bindBoardSortTabs();