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/lib/wxpay.js

@@ -142,10 +142,8 @@ async function verifyNotifySignature(headers, rawBody) {
       throw new Error('notify signature verification failed')
     }
   } catch (err) {
-    if (err.message.indexOf('unknown platform certificate') !== -1 || err.message.indexOf('signature verification') !== -1) {
-      throw err
-    }
-    console.error('[WXPAY] platform cert verification fallback:', err.message)
+    console.error('[WXPAY] signature verification failed:', err.message)
+    throw err
   }
   return true
 }