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/Admin/Schema/SchemaUsers.php

@@ -515,8 +515,6 @@ class SchemaUsers {
                     'groups'     => [
                         ['key' => 'basic', 'title' => '积分基础', 'fields' => [
                             ['key' => 'points_s', 'label' => '启用积分系统', 'type' => 'switch', 'default' => true],
-                            ['key' => 'points_pay_s', 'label' => '积分支付', 'type' => 'switch', 'default' => false],
-                            ['key' => 'points_rate', 'label' => '积分兑换比例', 'type' => 'number', 'default' => 100, 'min' => 1, 'max' => 10000],
                             ['key' => 'points_name', 'label' => '积分名称', 'type' => 'input', 'default' => '积分'],
                             ['key' => 'points_decimal', 'label' => '积分小数位数', 'type' => 'number', 'default' => 2, 'min' => 0, 'max' => 4],
                         ]],