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/app/api/alipay/notify/route.ts

@@ -0,0 +1,26 @@
+import { NextRequest } from 'next/server';
+import { handlePaymentNotify } from '@/lib/order/service';
+import { AlipayProvider } from '@/lib/alipay/provider';
+
+const alipayProvider = new AlipayProvider();
+
+export async function POST(request: NextRequest) {
+  try {
+    const rawBody = await request.text();
+    const headers: Record<string, string> = {};
+    request.headers.forEach((value, key) => {
+      headers[key] = value;
+    });
+
+    const notification = await alipayProvider.verifyNotification(rawBody, headers);
+    const success = await handlePaymentNotify(notification, alipayProvider.name);
+    return new Response(success ? 'success' : 'fail', {
+      headers: { 'Content-Type': 'text/plain' },
+    });
+  } catch (error) {
+    console.error('Alipay notify error:', error);
+    return new Response('fail', {
+      headers: { 'Content-Type': 'text/plain' },
+    });
+  }
+}