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.

inc/helpers/aksrt-credits.php

@@ -56,6 +56,10 @@ if (!function_exists('aksrt_credits_label')) {
             'free_post_favorite'=> __('文章被收藏', 'aksrt'),
             'free_comment_new'=> __('发表评论', 'aksrt'),
             'free_comment_like'=> __('评论被赞', 'aksrt'),
+            'points_exchange_in' => __('余额兑换积分', 'aksrt'),
+            'transfer_in' => __('积分转入', 'aksrt'),
+            'transfer_out' => __('积分转出', 'aksrt'),
+            'transfer_refund' => __('积分转账退款', 'aksrt'),
         ];
         if (isset($map[$reason])) {
             return $map[$reason];