fix: add HTTP timeout and cert cache error recovery in wxpay

- httpsRequest: 15s timeout, prevents hanging on WeChat API outage
- fetchPlatformCertificates: keeps old certs if refresh fails,
  only throws if no certs at all (first time failure)

server/src/lib/wxpay.js

@@ -80,6 +80,7 @@ function httpsRequest(method, path, body) {
         }
       })
     })
+    req.setTimeout(15000, () => { req.destroy(new Error('wxpay request timeout (15s)')) })
     req.on('error', reject)
     if (bodyStr) req.write(bodyStr)
     req.end()
@@ -117,6 +118,7 @@ let _platformCertsExpiry = 0
 
 async function fetchPlatformCertificates() {
   if (_platformCertsExpiry > Date.now()) return _platformCerts
+  try {
     const path = '/v3/certificates'
     const result = await httpsRequest('GET', path)
     const certs = {}
@@ -126,9 +128,15 @@ async function fetchPlatformCertificates() {
       const certPem = decryptResource(resource)
       certs[item.serial_no] = certPem
     }
+    if (Object.keys(certs).length > 0) {
       _platformCerts = certs
       _platformCertsExpiry = Date.now() + 12 * 3600 * 1000
-  return certs
+    }
+  } catch (err) {
+    console.error('[WXPAY] cert refresh failed, keeping old certs:', err.message)
+    if (Object.keys(_platformCerts).length === 0) throw err
+  }
+  return _platformCerts
 }
 
 async function verifyNotifySignature(headers, rawBody) {