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/subscribe-plans/subscribe-plans.js

@@ -91,6 +91,10 @@ Page({
     }).catch(function () {})
   },
 
+  onAgreement: function () {
+    wx.showModal({ title: '提示', content: '订阅协议内容建设中', showCancel: false })
+  },
+
   onSelect: function (e) {
     this.setData({ selected: e.currentTarget.dataset.plan })
   },

miniprogram/pages/subscribe-plans/subscribe-plans.wxml

@@ -40,6 +40,6 @@
     <button class="btn-primary btn-primary-gold" bindtap="onPurchase" disabled="{{purchasing}}" loading="{{purchasing}}">
       {{subscription && subscription.status === 'active' && subRemaining > 0 ? '续费' : '确认支付'}}
     </button>
-    <view class="agreement">支付即表示同意《订阅协议》</view>
+    <view class="agreement" bindtap="onAgreement">支付即表示同意《订阅协议》</view>
   </view>
 </view>