fix: mock purchase now actually activates subscription

POST /api/v1/subscription/purchase only creates an order without
activating. Add POST /api/v1/subscription/mock-purchase that does
purchase + verify in one step (non-production only). Miniprogram
subscribe page now calls mock-purchase so subscriptions take effect.

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

@@ -147,7 +147,7 @@ Page({
   doPurchase: function (plan) {
     var self = this
     self.setData({ purchasing: true })
-    api.purchase(plan.key).then(function (order) {
+    api.mockPurchase(plan.key).then(function (order) {
       self.setData({ purchasing: false })
       wx.showToast({ title: '支付成功', icon: 'success' })
       setTimeout(function () {