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

server/src/config.js

@@ -30,6 +30,15 @@ const config = {
   admin: {
     username: process.env.ADMIN_USERNAME || 'admin',
     password: process.env.ADMIN_PASSWORD || 'admin'
+  },
+  wxpay: {
+    appid: process.env.WECHAT_APPID,
+    mchId: process.env.WX_MCH_ID || '',
+    apiV3Key: process.env.WX_MCH_API_V3_KEY || '',
+    mchSerialNo: process.env.WX_MCH_SERIAL_NO || '',
+    privateKey: process.env.WX_MCH_PRIVATE_KEY || '',
+    privateKeyPath: process.env.WX_MCH_PRIVATE_KEY_PATH || '',
+    notifyUrl: process.env.WX_PAY_NOTIFY_URL || ''
   }
 }