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>

Dockerfile

@@ -21,7 +21,6 @@ COPY --from=build /build/cameleer3-server-app/target/cameleer3-server-app-*.jar
 ENV SPRING_DATASOURCE_URL=jdbc:postgresql://postgres:5432/cameleer3
 ENV SPRING_DATASOURCE_USERNAME=cameleer
 ENV SPRING_DATASOURCE_PASSWORD=cameleer_dev
-ENV OPENSEARCH_URL=http://opensearch:9200
 
 EXPOSE 8081
 ENV TZ=UTC