feat: fetch subscription prices from server settings

- Add GET /api/v1/subscription/plans public endpoint (no auth needed)
  that reads prices from system_settings table
- Subscribe-plans page now loads prices from server on show
- Falls back to hardcoded defaults if API fails
- Add api.getPlans() to miniprogram API module

miniprogram/utils/api.js

@@ -17,6 +17,7 @@ module.exports = {
   unbindDevice: function (deviceId) { return http.post('/api/v1/device/unbind', deviceId ? { device_id: deviceId } : {}) },
 
   // Subscription
+  getPlans: function () { return http.get('/api/v1/subscription/plans') },
   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 }) },