feat: add password change, trial subscription, batch import, UX improvements

Server:
- POST /api/v1/admin/password: admin password change with bcrypt migration
- POST /api/v1/subscription/trial: user trial activation, one per user
- POST /api/v1/admin/devices/batch: bulk device import (up to 500)
- Add trial plan (7 days, free) to PLANS constant

Admin console:
- Settings page: add password change form with validation
- Device page: add batch import modal with textarea input

Miniprogram:
- Treating page: add back button with stop-treatment confirmation
- Index page: add mock device bind button (dev mode only)

miniprogram/pages/treating/treating.js

@@ -178,6 +178,10 @@ Page({
     })
   },
 
+  onBack: function () {
+    this.onStop()
+  },
+
   onStop: function () {
     var self = this
     wx.showModal({