feat: remove OpenSearch, add ClickHouse admin page

Remove all OpenSearch code, dependencies, configuration, deployment
manifests, and CI/CD references. Replace the OpenSearch admin page
with a ClickHouse admin page showing cluster status, table sizes,
performance metrics, and indexer pipeline stats.

- Delete 11 OpenSearch Java files (config, search impl, admin controller, DTOs, tests)
- Delete 3 OpenSearch frontend files (admin page, CSS, query hooks)
- Delete deploy/opensearch.yaml K8s manifest
- Remove opensearch Maven dependencies from pom.xml
- Remove opensearch config from application.yml, Dockerfile, docker-compose
- Remove opensearch from CI workflow (secrets, deploy, cleanup steps)
- Simplify ThresholdConfig (remove OpenSearch thresholds, database-only)
- Change default search backend from opensearch to clickhouse
- Add ClickHouseAdminController with /status, /tables, /performance, /pipeline
- Add ClickHouseAdminPage with StatCards, pipeline ProgressBar, tables DataTable
- Update CLAUDE.md, HOWTO.md, and source comments

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/router.tsx

@@ -14,7 +14,7 @@ const RbacPage = lazy(() => import('./pages/Admin/RbacPage'));
 const AuditLogPage = lazy(() => import('./pages/Admin/AuditLogPage'));
 const OidcConfigPage = lazy(() => import('./pages/Admin/OidcConfigPage'));
 const DatabaseAdminPage = lazy(() => import('./pages/Admin/DatabaseAdminPage'));
-const OpenSearchAdminPage = lazy(() => import('./pages/Admin/OpenSearchAdminPage'));
+const ClickHouseAdminPage = lazy(() => import('./pages/Admin/ClickHouseAdminPage'));
 const AppConfigPage = lazy(() => import('./pages/Admin/AppConfigPage'));
 const SwaggerPage = lazy(() => import('./pages/Swagger/SwaggerPage'));
 
@@ -87,7 +87,7 @@ export const router = createBrowserRouter([
               { path: 'oidc', element: <SuspenseWrapper><OidcConfigPage /></SuspenseWrapper> },
               { path: 'appconfig', element: <SuspenseWrapper><AppConfigPage /></SuspenseWrapper> },
               { path: 'database', element: <SuspenseWrapper><DatabaseAdminPage /></SuspenseWrapper> },
-              { path: 'opensearch', element: <SuspenseWrapper><OpenSearchAdminPage /></SuspenseWrapper> },
+              { path: 'clickhouse', element: <SuspenseWrapper><ClickHouseAdminPage /></SuspenseWrapper> },
             ],
           },
           { path: 'api-docs', element: <SuspenseWrapper><SwaggerPage /></SuspenseWrapper> },