feat: add digital avatar management to admin dashboard

- Add avatars list page with pagination, search, status filter
- Add avatar detail dialog showing full info and config params
- Add on/off switch (active/inactive status toggle)
- Backend: avatar_service.py connects to digital avatar SQLite
- Backend: /api/avatars endpoints for list, detail, status update
- Sidebar menu item added below dashboard
- docker-compose: mount SQLite db and set AVATAR_DB_PATH env var

frontend/src/layouts/MainLayout.vue

@@ -11,6 +11,10 @@
           <el-icon><DataAnalysis /></el-icon>
           <span>数据看板</span>
         </el-menu-item>
+        <el-menu-item index="/avatars">
+          <el-icon><UserFilled /></el-icon>
+          <span>数字分身管理</span>
+        </el-menu-item>
         <el-menu-item index="/users">
           <el-icon><User /></el-icon>
           <span>虚拟用户</span>