merge: 合并 wxpay_direct 微信支付直连分支

解决冲突：保留 main 的常量化/provider 字段/ENABLED_PAYMENT_TYPES 移除，
合并 worktree 的微信支付直连实现、notifyUrl/returnUrl 传递、签名验证优化。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/orders/route.ts

@@ -47,9 +47,9 @@ export async function POST(request: NextRequest) {
       amount,
       paymentType: payment_type,
       clientIp,
+      isMobile: is_mobile,
       srcHost: src_host,
       srcUrl: src_url,
-      isMobile: is_mobile,
     });
 
     // 不向客户端暴露 userName / userBalance 等隐私字段

src/app/api/wxpay/notify/route.ts

@@ -0,0 +1,31 @@
+import { NextRequest } from 'next/server';
+import { handlePaymentNotify } from '@/lib/order/service';
+import { WxpayProvider } from '@/lib/wxpay';
+
+const wxpayProvider = new WxpayProvider();
+
+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 wxpayProvider.verifyNotification(rawBody, headers);
+    if (!notification) {
+      return Response.json({ code: 'SUCCESS', message: '成功' });
+    }
+    const success = await handlePaymentNotify(notification, wxpayProvider.name);
+    return Response.json(
+      success ? { code: 'SUCCESS', message: '成功' } : { code: 'FAIL', message: '处理失败' },
+      { status: success ? 200 : 500 },
+    );
+  } catch (error) {
+    console.error('Wxpay notify error:', error);
+    return Response.json(
+      { code: 'FAIL', message: '处理失败' },
+      { status: 500 },
+    );
+  }
+}