feat(credits): exchange balance for points

Replace the legacy points-to-balance and external points purchase flows
with direct integer balance-to-points exchange. Enforce exchange and
points-transfer limits and capability checks across the UI and REST API.

src/Payment/UserOrders.php

@@ -313,9 +313,6 @@ class UserOrders {
         if ($type === DomainModel::ORDER_PRODUCT_RECHARGE) {
             return __('余额充值', 'aksrt');
         }
-        if ($type === DomainModel::ORDER_PRODUCT_POINTS_BUY) {
-            return __('购买积分', 'aksrt');
-        }
         if ($product_id > 0) {
             $cache_key = (string) $product_id;
             if (!array_key_exists($cache_key, $this->post_title_cache)) {
@@ -357,7 +354,7 @@ class UserOrders {
 
         if ($type === DomainModel::ORDER_PRODUCT_VIP) {
             $url = function_exists('aksrt_get_user_center_tab_url') ? aksrt_get_user_center_tab_url('vip') : '';
-        } elseif (in_array($type, [DomainModel::ORDER_PRODUCT_RECHARGE, DomainModel::ORDER_PRODUCT_POINTS_BUY], true)) {
+        } elseif ($type === DomainModel::ORDER_PRODUCT_RECHARGE) {
             $url = function_exists('aksrt_get_user_center_tab_url') ? aksrt_get_user_center_tab_url('credits') : '';
         } else {
             $url = $post_id > 0 ? get_permalink($post_id) : '';
@@ -371,7 +368,6 @@ class UserOrders {
         return match ($type) {
             DomainModel::ORDER_PRODUCT_VIP => __('查看会员权益', 'aksrt'),
             DomainModel::ORDER_PRODUCT_RECHARGE => __('查看余额', 'aksrt'),
-            DomainModel::ORDER_PRODUCT_POINTS_BUY => __('查看积分', 'aksrt'),
             default => __('打开已购内容', 'aksrt'),
         };
     }