feat: Prometheus数据源管理 — 参考Grafana界面CRUD

- 后端: PrometheusDataSource model (22字段，含Basic Auth/TLS/Headers/Alerting)
- 后端: 8 API (CRUD + 设默认 + 连接测试 + 审计)
- 前端: 数据源管理页 — 表格列表 + Grafana风格配置Dialog
- 前端: 侧边栏「监控中心→数据源」菜单项
- 功能: Save & test 连接测试（含Basic Auth + Skip TLS）

Author: Logan.Li

frontend/src/App.tsx

@@ -33,6 +33,7 @@ import TraceManager from '@/components/monitor/trace/TraceManager';
 import HostMonitorDashboard from '@/components/monitor/dashboard/HostMonitorDashboard';
 import { MonitorCenter } from '@/pages/MonitorCenter';
 import { MetricsExplorer } from '@/pages/MetricsExplorer';
+import PrometheusDatasourceManager from '@/components/monitor/datasource/PrometheusDatasourceManager';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -74,6 +75,7 @@ function App() {
             <Route path="alert-center" element={<AlertCenter />} />
             <Route path="trace" element={<TraceManager />} />
             <Route path="dashboards" element={<HostMonitorDashboard />} />
+            <Route path="datasources" element={<PrometheusDatasourceManager />} />
             <Route path="alerts" element={<AlertManager />} />
             <Route path="logs" element={<AlertManager />} />
             <Route path="approval" element={<ApprovalFlowManager />} />