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/lib/alipay/client.ts

@@ -77,6 +77,7 @@ export async function execute<T extends AlipayResponse>(
     method: 'POST',
     headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
     body: new URLSearchParams(params).toString(),
+    signal: AbortSignal.timeout(10_000),
   });
 
   const data = await response.json();