feat: WeChat Pay V3 integration (fill credentials to activate)

Server:
- New lib/wxpay.js: native crypto RSA-SHA256 signing, JSAPI prepay,
  AES-256-GCM notify decryption, order query (no npm deps)
- New dao/payment-order.dao.js: createOrder, markPrepay,
  markPaidAndActivateSubscription (idempotent + transactional)
- New routes/payment.js: GET order status, POST order sync
- New routes/payment-notify.js: WeChat async callback handler with
  signature verification, amount/appid/mchid validation
- Modified subscription/purchase: auto-detects wxpay config, returns
  real payment_params or mock fallback
- Schema: payment_orders table with out_trade_no unique key
- app.js: express.raw() for notify path, payment routes mounted
- config.js: wxpay block with 7 env vars
- .env.example: all WeChat Pay fields documented
- .gitignore: certs/, *.pem, *.p12

Miniprogram:
- subscribe-plans doPurchase: calls real purchase API, falls back to
  mockPurchase only when server returns mock:true
- Added syncAndRedirect for post-payment order confirmation
- api.js: getPaymentOrder, syncPaymentOrder
- Removed "模拟支付"/"测试环境" from UI text

miniprogram/utils/api.js

@@ -23,6 +23,10 @@ module.exports = {
   purchase: function (plan) { return http.post('/api/v1/subscription/purchase', { plan: plan }) },
   mockPurchase: function (plan) { return http.post('/api/v1/subscription/mock-purchase', { plan: plan }) },
 
+  // Payment
+  getPaymentOrder: function (orderId) { return http.get('/api/v1/payment/orders/' + orderId) },
+  syncPaymentOrder: function (orderId) { return http.post('/api/v1/payment/orders/' + orderId + '/sync') },
+
   // Treatment
   getRecords: function (params) { return http.get('/api/v1/treatment/history', params) },
   syncTreatment: function (data) { return http.post('/api/v1/treatment/sync', data) },