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/utils/api.js

@@ -21,6 +21,7 @@ module.exports = {
   getSubscription: function () { return http.get('/api/v1/subscription') },
   activateTrial: function () { return http.post('/api/v1/subscription/trial') },
   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 }) },
 
   // Treatment
   getRecords: function (params) { return http.get('/api/v1/treatment/history', params) },