feat: Phase 2-5 监控中心全栈实现 — 指标浏览器/SNMP/站点拨测/中间件

Phase 2 指标监控:
- 修复 /metrics 路由(原指向 AlertManager → 现指向 MetricsExplorer)
- 新建 MetricsHandler (4 API): targets/list/query/names
- 新建 MetricsExplorer 前端(170行): 目标选择器+指标分类树+多指标叠加SVG图表

Phase 3 SNMP设备:
- 新建 SNMPHandler (9 API): devices CRUD/templates/stats
- SNMPMonitor 接入真实API(428行): 设备表格+添加对话框+删除

Phase 4 站点拨测:
- 新建 SiteHandler (8 API): tasks CRUD/results/stats
- SiteMonitor 接入真实API(542行): 任务表+添加对话框+拨测类型网格
- 新增 siteApi 到 api.ts

Phase 5 中间件采集:
- Agent 新增 collector/middleware.go: Redis INFO + MySQL handshake 采集
- Agent CLI 新增 -redis/-redis-auth/-mysql/-mysql-user/-mysql-pass
- MiddlewareMonitor 接入真实API(301行): 实例卡片+指标面板

基础设施:
- AutoMigrate 新增 23 张监控表
- 前后端容器构建部署验证通过

frontend/src/services/api.ts

@@ -648,6 +648,25 @@ export const alertCenterApi = {
   },
 };
 
+// ==================== 站点拨测 API ====================
+export const siteApi = {
+  getStats: async () => {
+    return http.get('/site/stats');
+  },
+  getTasks: async () => {
+    return http.get<{ list: any[]; total: number }>('/site/tasks');
+  },
+  createTask: async (data: { name: string; url: string; type: string; frequency_sec?: number; timeout_sec?: number; expected_status_code?: number }) => {
+    return http.post('/site/tasks', data);
+  },
+  deleteTask: async (id: number | string) => {
+    return http.delete(`/site/tasks/${id}`);
+  },
+  toggleTask: async (id: number | string, enabled: boolean) => {
+    return http.put(`/site/tasks/${id}`, { enabled });
+  },
+};
+
 // ==================== 链路追踪 API（暂无后端，返回空结果优雅降级） ====================
 export const traceApi = {
   getTraces: async (_params: any) => ({ data: [], total: 0 }),