feat(monitor): F4 自定义监控仪表盘 — mock 桩改真实 CRUD + PromQL 查询渲染

后端:
- 新增 model/custom_dashboard.go (monitor_custom_dashboards 表)
- 新增 controller/custom_dashboard.go (CRUD + 设默认, 6 路由)
- routes.go 注册 /custom-dashboards 路由组
- database.go AutoMigrate 注册 CustomDashboard

前端:
- grafana.ts: dashboardApi 从 mock 返回空改为真实 HTTP 调用
- metrics.ts: prometheusProxyApi 新增 query/queryRange 方法
- GrafanaPanelContent.tsx: 从随机 mock 数据改为执行 PromQL 查询
  + Recharts LineChart 渲染 graph 类型
  + SVG gauge 渲染 gauge 类型
  + 数值卡片渲染 stat 类型
  + 表格渲染 table 类型
  + 30s 自动刷新 + loading/error 状态
  + 自动发现默认 Prometheus 数据源
- GrafanaDashboard.tsx: 传递 datasourceUrl 到 GrafanaPanelContent

验证: go build✅ npm build✅ 17/17 集成测试通过 E2E CRUD 全通过

backend/router/routes.go

@@ -966,6 +966,18 @@ func Register(r *gin.Engine, db *gorm.DB, log *zap.Logger, cfg *config.Config, c
 		zabbixDSGroup.POST("/test", zabbixDSHandler.TestConnection)
 	}
 
+	// 自定义监控仪表盘 — CRUD + 设默认 // Logan.Li
+	customDashHandler := controller.NewCustomDashboardHandler(db)
+	customDashGroup := authorized.Group("/custom-dashboards")
+	{
+		customDashGroup.GET("", customDashHandler.List)
+		customDashGroup.GET("/:id", customDashHandler.Get)
+		customDashGroup.POST("", customDashHandler.Create)
+		customDashGroup.PUT("/:id", customDashHandler.Update)
+		customDashGroup.DELETE("/:id", customDashHandler.Delete)
+		customDashGroup.POST("/:id/default", customDashHandler.SetDefault)
+	}
+
 	// 凭据管理 — SSH密钥/API Token/密码统一加密管理 // Logan.Li
 	credentialHandlerV2 := controller.NewCredentialHandlerV2(svcs.Credential)
 	credentialHandler := controller.NewCredentialHandler(db, cfg.JWT.Secret) // 仅用于 GetTags/PatchCredential/AssignCredential（待拆分）