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/App.tsx

@@ -32,6 +32,7 @@ import AlertCenter from '@/components/monitor/alert-center/AlertCenter';
 import TraceManager from '@/components/monitor/trace/TraceManager';
 import GrafanaDashboard from '@/components/monitor/dashboard/GrafanaDashboard';
 import { MonitorCenter } from '@/pages/MonitorCenter';
+import { MetricsExplorer } from '@/pages/MetricsExplorer';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -69,7 +70,7 @@ function App() {
             <Route path="releases" element={<ReleaseManager />} />
             <Route path="deploy" element={<AppReleaseManager />} />
             {/* 监控告警 */}
-            <Route path="metrics" element={<AlertManager />} />
+            <Route path="metrics" element={<MetricsExplorer />} />
             <Route path="alert-center" element={<AlertCenter />} />
             <Route path="trace" element={<TraceManager />} />
             <Route path="grafana" element={<GrafanaDashboard />} />