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/lib/wxpay.js

@@ -8,9 +8,11 @@ function isConfigured() {
   return !!(c.mchId && c.apiV3Key && c.mchSerialNo && (c.privateKey || c.privateKeyPath))
 }
 
+let _cachedPrivateKey = null
 function getPrivateKey() {
-  if (config.wxpay.privateKey) return config.wxpay.privateKey
-  if (config.wxpay.privateKeyPath) return fs.readFileSync(config.wxpay.privateKeyPath, 'utf8')
+  if (_cachedPrivateKey) return _cachedPrivateKey
+  if (config.wxpay.privateKey) { _cachedPrivateKey = config.wxpay.privateKey; return _cachedPrivateKey }
+  if (config.wxpay.privateKeyPath) { _cachedPrivateKey = fs.readFileSync(config.wxpay.privateKeyPath, 'utf8'); return _cachedPrivateKey }
   throw new Error('WeChat Pay private key not configured')
 }