feat: 添加安全仪表板前端页面

- 新增 SecurityPage.vue: 统计卡片、威胁事件表格、封禁管理、风险查询
- 新增 api/security.js: 安全相关API封装
- 路由添加 /security 页面
- 侧边栏添加"安全防护"菜单项

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

admin-frontend/src/layouts/AdminLayout.vue

@@ -7,6 +7,7 @@ import {
   ChatLineSquare,
   Document,
   List,
+  Lock,
   Message,
   Setting,
   Tools,
@@ -104,6 +105,7 @@ const menuItems = [
   { path: '/logs', label: '任务日志', icon: List },
   { path: '/announcements', label: '公告', icon: Bell },
   { path: '/email', label: '邮件', icon: Message },
+  { path: '/security', label: '安全防护', icon: Lock },
   { path: '/system', label: '系统配置', icon: Tools },
   { path: '/settings', label: '设置', icon: Setting },
 ]