feat: 支付手续费功能

- 支持提供商级别和渠道级别手续费率配置（FEE_RATE_PROVIDER_* / FEE_RATE_*）
- 用户多付手续费，到账金额不变（充值 ¥100 + 1.6% = 实付 ¥101.60）
- 前端显示手续费明细和实付金额
- 退款时按实付金额退款，余额扣减到账金额

src/lib/order/fee.ts

@@ -0,0 +1,38 @@
+import { initPaymentProviders, paymentRegistry } from '@/lib/payment';
+
+/**
+ * 获取指定支付渠道的手续费率（百分比）。
+ * 优先级：FEE_RATE_{TYPE} > FEE_RATE_PROVIDER_{KEY} > 0
+ */
+export function getMethodFeeRate(paymentType: string): number {
+  // 渠道级别：FEE_RATE_ALIPAY / FEE_RATE_WXPAY / FEE_RATE_STRIPE
+  const methodRaw = process.env[`FEE_RATE_${paymentType.toUpperCase()}`];
+  if (methodRaw !== undefined && methodRaw !== '') {
+    const num = Number(methodRaw);
+    if (Number.isFinite(num) && num >= 0) return num;
+  }
+
+  // 提供商级别：FEE_RATE_PROVIDER_EASYPAY / FEE_RATE_PROVIDER_STRIPE
+  initPaymentProviders();
+  const providerKey = paymentRegistry.getProviderKey(paymentType);
+  if (providerKey) {
+    const providerRaw = process.env[`FEE_RATE_PROVIDER_${providerKey.toUpperCase()}`];
+    if (providerRaw !== undefined && providerRaw !== '') {
+      const num = Number(providerRaw);
+      if (Number.isFinite(num) && num >= 0) return num;
+    }
+  }
+
+  return 0;
+}
+
+/**
+ * 根据到账金额和手续费率计算实付金额。
+ * feeAmount = ceil(rechargeAmount * feeRate / 100 * 100) / 100  （进一制到分）
+ * payAmount = rechargeAmount + feeAmount
+ */
+export function calculatePayAmount(rechargeAmount: number, feeRate: number): number {
+  if (feeRate <= 0) return rechargeAmount;
+  const feeAmount = Math.ceil(rechargeAmount * feeRate / 100 * 100) / 100;
+  return Math.round((rechargeAmount + feeAmount) * 100) / 100;
+}