fix: 5 critical payment issues from code review

1. Move notify route before authMiddleware (WeChat callback has no JWT)
2. Add await to verifyNotifySignature call (was fire-and-forget)
3. Remove dangerous verify fallback — all signature failures now throw
4. Payment sync polls 3x before giving up, never redirects to success
   page unless confirmed paid
5. Production guard enforces all WX_MCH_* env vars on startup

server/src/routes/payment-notify.js

@@ -9,7 +9,7 @@ async function handleNotify(req, res) {
     const rawBody = typeof req.body === 'string' ? req.body : (Buffer.isBuffer(req.body) ? req.body.toString('utf8') : JSON.stringify(req.body))
     const parsed = typeof req.body === 'object' && !Buffer.isBuffer(req.body) ? req.body : JSON.parse(rawBody)
 
-    wxpay.verifyNotifySignature(req.headers, rawBody)
+    await wxpay.verifyNotifySignature(req.headers, rawBody)
 
     const result = wxpay.decryptNotifyResource(parsed.resource)