feat: complete feature gaps across all modules

Server:
- Add settings-cache with 60s TTL for feature toggle checks
- Enforce maintenance_mode on login, enable_binding on device bind

Admin console:
- Remove dead "发送通知" button from user detail
- Firmware check calls real API and compares versions

Miniprogram:
- Wear-check: dynamic battery/connected from BLE state
- Login: hide non-functional phone auth button
- Treating: show actual selected regions instead of hardcoded "全脸"
- Index: display subscription status with tap to manage
- Auto-scan: show "待检测" instead of "--" for PD data
- Agreements: tap shows "内容建设中" modal

miniprogram/pages/auto-scan/auto-scan.wxml

@@ -15,6 +15,7 @@
     </view>
 
     <view class="scan-hint">请将面罩对准面部</view>
+    <view class="scan-hint" style="font-size:24rpx;color:#999;margin-top:8rpx;">连接设备后可获取皮肤数据</view>
 
     <view class="debug-actions" wx:if="{{devMode}}">
       <view class="debug-label">🔧 调试工具</view>