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/pages/subscribe-plans/subscribe-plans.js

@@ -130,8 +130,8 @@ Page({
     var planDays = plan.key === 'yearly' ? 365 : 30
     var title = isRenew ? '确认续费' : '确认支付'
     var content = isRenew
-      ? '在现有订阅基础上延长' + planDays + '天，模拟支付 ¥' + plan.price + '？（测试环境）'
-      : '模拟支付 ¥' + plan.price + '？（测试环境）'
+      ? '在现有订阅基础上延长' + planDays + '天，支付 ¥' + plan.price
+      : '支付 ¥' + plan.price
 
     wx.showModal({
       title: title,
@@ -162,15 +162,65 @@ Page({
   doPurchase: function (plan) {
     var self = this
     self.setData({ purchasing: true })
-    api.mockPurchase(plan.key).then(function (order) {
-      self.setData({ purchasing: false })
-      wx.showToast({ title: '支付成功', icon: 'success' })
-      setTimeout(function () {
-        wx.redirectTo({ url: '/pages/subscribe-success/subscribe-success?plan=' + plan.key })
-      }, 1000)
+    api.purchase(plan.key).then(function (data) {
+      if (data.mock || !data.payment_params) {
+        // Dev fallback: use mock purchase
+        return api.mockPurchase(plan.key).then(function () {
+          self.setData({ purchasing: false })
+          wx.showToast({ title: '支付成功', icon: 'success' })
+          setTimeout(function () {
+            wx.redirectTo({ url: '/pages/subscribe-success/subscribe-success?plan=' + plan.key })
+          }, 1000)
+        })
+      }
+      // Real payment
+      var params = data.payment_params
+      self._currentOrderId = data.order_id
+      wx.requestPayment({
+        timeStamp: params.timeStamp,
+        nonceStr: params.nonceStr,
+        package: params.package,
+        signType: params.signType,
+        paySign: params.paySign,
+        success: function () {
+          // Payment dialog succeeded, sync order to confirm
+          self.syncAndRedirect(data.order_id, plan.key)
+        },
+        fail: function (err) {
+          self.setData({ purchasing: false })
+          var msg = (err.errMsg || '').indexOf('cancel') > -1 ? '已取消支付' : '支付失败'
+          wx.showToast({ title: msg, icon: 'none' })
+        }
+      })
     }).catch(function (err) {
       self.setData({ purchasing: false })
-      wx.showToast({ title: err.message || '支付失败', icon: 'none' })
+      wx.showToast({ title: err.message || '创建订单失败', icon: 'none' })
+    })
+  },
+
+  syncAndRedirect: function (orderId, planKey) {
+    var self = this
+    api.syncPaymentOrder(orderId).then(function (result) {
+      self.setData({ purchasing: false })
+      if (result.status === 'paid') {
+        wx.showToast({ title: '支付成功', icon: 'success' })
+        setTimeout(function () {
+          wx.redirectTo({ url: '/pages/subscribe-success/subscribe-success?plan=' + planKey })
+        }, 1000)
+      } else {
+        // Callback may not have arrived yet, still redirect optimistically
+        wx.showToast({ title: '支付处理中', icon: 'none' })
+        setTimeout(function () {
+          wx.redirectTo({ url: '/pages/subscribe-success/subscribe-success?plan=' + planKey })
+        }, 2000)
+      }
+    }).catch(function () {
+      self.setData({ purchasing: false })
+      // Even if sync fails, payment may still succeed via callback
+      wx.showToast({ title: '支付处理中，请稍后查看', icon: 'none' })
+      setTimeout(function () {
+        wx.redirectTo({ url: '/pages/subscribe-success/subscribe-success?plan=' + planKey })
+      }, 2000)
     })
   }
 })