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/index/index.wxml

@@ -4,6 +4,10 @@
       <text class="empty-icon">📱</text>
       <view class="empty-text">还没有绑定设备</view>
       <button class="btn-primary mt-30" bindtap="onAddDevice">扫码添加设备</button>
+      <view class="debug-actions" wx:if="{{devMode}}">
+        <view class="debug-label">🔧 调试工具</view>
+        <button class="btn-secondary btn-sm" bindtap="onMockBind">模拟绑定设备</button>
+      </view>
     </view>
   </view>