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();

src/lib/alipay/provider.ts

@@ -82,14 +82,7 @@ export class AlipayProvider implements PaymentProvider {
     }
 
     const sign = params.sign || '';
-    const paramsForVerify: Record<string, string> = {};
-    for (const [key, value] of Object.entries(params)) {
-      if (key !== 'sign' && key !== 'sign_type' && value !== undefined && value !== null) {
-        paramsForVerify[key] = value;
-      }
-    }
-
-    if (!env.ALIPAY_PUBLIC_KEY || !verifySign(paramsForVerify, env.ALIPAY_PUBLIC_KEY, sign)) {
+    if (!env.ALIPAY_PUBLIC_KEY || !verifySign(params, env.ALIPAY_PUBLIC_KEY, sign)) {
       throw new Error('Alipay notification signature verification failed');
     }