feat(payment): tighten gateways and implement Epay

Keep only balance, Alipay, WeChat, PayPal, Stripe, Epay, and card-pass
channels; remove placeholder aggregators. Add dual-channel Epay gateway
with MD5 notify/return, register cashier methods, and gate card-pass
redeem behind the payment channel switch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AKSRT/Payment/UserOrders.php

@@ -504,13 +504,15 @@ class UserOrders {
     private function payment_method_label(string $method): string {
         $method = sanitize_key($method);
         $map = [
-            'balance'    => __('站内余额', 'aksrt'),
-            'points'     => __('积分支付', 'aksrt'),
-            'alipay'     => __('支付宝', 'aksrt'),
-            'wechatpay'  => __('微信支付', 'aksrt'),
-            'wechat'     => __('微信支付', 'aksrt'),
-            'paypal'     => 'PayPal',
-            'stripe'     => 'Stripe',
+            'balance'     => __('站内余额', 'aksrt'),
+            'points'      => __('积分支付', 'aksrt'),
+            'alipay'      => __('支付宝', 'aksrt'),
+            'wechatpay'   => __('微信支付', 'aksrt'),
+            'wechat'      => __('微信支付', 'aksrt'),
+            'paypal'      => 'PayPal',
+            'stripe'      => 'Stripe',
+            'epay_alipay' => __('易支付·支付宝', 'aksrt'),
+            'epay_wxpay'  => __('易支付·微信', 'aksrt'),
         ];
         return $map[$method] ?? ($method !== '' ? $method : __('—', 'aksrt'));
     }