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-content-pay.php

@@ -1056,35 +1056,13 @@ class AKSRT_Content_Pay {
             ];
         }
 
-        $methods = [];
-        $ids = $logged_in
-            ? ['balance', 'alipay', 'wechatpay', 'paypal', 'stripe']
-            : ['alipay', 'wechatpay', 'paypal', 'stripe'];
-        if (!class_exists('AKSRT_Payment_Gateway')) {
-            return $logged_in
-                ? [['id' => 'balance', 'name' => __('站内余额', 'aksrt')]]
-                : [['id' => 'alipay', 'name' => __('支付宝', 'aksrt')]];
+        if (function_exists('aksrt_list_cash_payment_methods')) {
+            return aksrt_list_cash_payment_methods($logged_in);
         }
-        $gw = AKSRT_Payment_Gateway::get_instance();
-        foreach ($ids as $id) {
-            $g = $gw->get_gateway($id);
-            if (!$g) {
-                continue;
-            }
-            if (method_exists($g, 'is_configured') && !$g->is_configured()) {
-                continue;
-            }
-            $methods[] = [
-                'id' => $id,
-                'name' => method_exists($g, 'get_name') ? (string) $g->get_name() : $id,
-            ];
-        }
-        if ($methods === []) {
-            $methods[] = $logged_in
-                ? ['id' => 'balance', 'name' => __('站内余额', 'aksrt')]
-                : ['id' => 'alipay', 'name' => __('支付宝', 'aksrt')];
-        }
-        return $methods;
+
+        return $logged_in
+            ? [['id' => 'balance', 'name' => __('站内余额', 'aksrt')]]
+            : [['id' => 'alipay', 'name' => __('支付宝', 'aksrt')]];
     }