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.wxml

@@ -1,5 +1,6 @@
 <view class="page">
   <view class="page-header page-header-blue" style="padding-top: {{statusBarHeight + 24}}px;">
+    <view class="nav-back nav-back-light" bindtap="onBack">‹ 返回</view>
     <view class="page-header-title">护理中</view>
     <view class="page-header-subtitle">正在护理...</view>
   </view>