fix: payment security hardening — 3 CRITICAL + 3 HIGH

CRITICAL fixes:
- C1: Notify amount validation now unconditional (was skippable if amount field missing)
- C2: Sync endpoint validates payer_total against order amount before activation
- C3: Order ID uses crypto.randomBytes(6) instead of Math.random (collision-safe)

HIGH fixes:
- H1: Payment endpoints rate limited to 5/min per IP
- H2: Max 5 pending orders per user, reject new ones until completed/cancelled
- H3: Purchase endpoint returns error (not mock) when wxpay unconfigured in production

Also fixed:
- Notify handler asserts Buffer body, rejects non-Buffer (L3)
- Notify error response is generic, no internal message leak (M1)
- Private key cached in memory after first read (L2)
- Fixed duplicate paymentOrderDao const declaration

server/src/app.js

@@ -42,10 +42,20 @@ app.use((req, res, next) => {
   next()
 })
 
+const paymentLimiter = rateLimit({
+  windowMs: 60 * 1000,
+  max: 5,
+  standardHeaders: true,
+  legacyHeaders: false,
+  message: { code: 2001, message: 'too_many_attempts' }
+})
+
 app.use('/api/v1/auth/login', userLoginLimiter)
 app.use('/api/v1/admin/login', adminLoginLimiter)
 app.use('/api/v1/user/avatar', uploadLimiter)
 app.use('/api/v1/user/phone', uploadLimiter)
+app.use('/api/v1/subscription/purchase', paymentLimiter)
+app.use('/api/v1/payment/orders', paymentLimiter)
 
 // WeChat Pay callback — must be before authMiddleware (no JWT)
 app.post('/api/v1/payment/wechat/notify', require('./routes/payment-notify'))