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/Core/DomainModel.php

@@ -28,8 +28,6 @@ class DomainModel {
     public const ORDER_PRODUCT_RESOURCE     = 'resource';
     public const ORDER_PRODUCT_VIP          = 'vip';
     public const ORDER_PRODUCT_RECHARGE     = 'recharge';
-    /** Cash purchase of points (zibll order_type=9). */
-    public const ORDER_PRODUCT_POINTS_BUY   = 'points_buy';
 
     /**
      * Human labels for aksrt_order.product_type (front-end order center).
@@ -41,7 +39,6 @@ class DomainModel {
             self::ORDER_PRODUCT_RESOURCE     => __('付费内容', 'aksrt'),
             self::ORDER_PRODUCT_VIP          => __('VIP 会员', 'aksrt'),
             self::ORDER_PRODUCT_RECHARGE     => __('余额充值', 'aksrt'),
-            self::ORDER_PRODUCT_POINTS_BUY   => __('购买积分', 'aksrt'),
         ];
     }