feat: 集成支付宝电脑网站支付（alipay direct）

- 新增 src/lib/alipay/ 模块：RSA2 签名、网关客户端、AlipayProvider
- 新增 /api/alipay/notify 异步通知回调路由
- config.ts 添加 ALIPAY_* 环境变量
- payment/index.ts 注册 alipaydirect 提供商
- 27 个单元测试全部通过

src/lib/payment/index.ts

@@ -2,6 +2,7 @@ import { paymentRegistry } from './registry';
 import type { PaymentType } from './types';
 import { EasyPayProvider } from '@/lib/easy-pay/provider';
 import { StripeProvider } from '@/lib/stripe/provider';
+import { AlipayProvider } from '@/lib/alipay/provider';
 import { getEnv } from '@/lib/config';
 
 export { paymentRegistry } from './registry';
@@ -31,6 +32,13 @@ export function initPaymentProviders(): void {
     paymentRegistry.register(new EasyPayProvider());
   }
 
+  if (providers.includes('alipaydirect')) {
+    if (!env.ALIPAY_APP_ID || !env.ALIPAY_PRIVATE_KEY) {
+      throw new Error('PAYMENT_PROVIDERS 含 alipaydirect，但缺少 ALIPAY_APP_ID 或 ALIPAY_PRIVATE_KEY');
+    }
+    paymentRegistry.register(new AlipayProvider());
+  }
+
   if (providers.includes('stripe')) {
     if (!env.STRIPE_SECRET_KEY) {
       throw new Error('PAYMENT_PROVIDERS 含 stripe，但缺少 STRIPE_SECRET_KEY');