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>

deploy/overlays/feature/kustomization.yaml

@@ -25,8 +25,6 @@ patches:
                 env:
                   - name: CAMELEER_DB_SCHEMA
                     value: "BRANCH_SCHEMA"
-                  - name: CAMELEER_OPENSEARCH_INDEX_PREFIX
-                    value: "cam-BRANCH_SLUG-executions-"
                   - name: CAMELEER_UI_ORIGIN
                     value: "http://BRANCH_SLUG.cameleer.siegeln.net"
   # UI ConfigMap: branch-specific API URL

deploy/overlays/main/kustomization.yaml

@@ -40,8 +40,6 @@ patches:
                 env:
                   - name: SPRING_DATASOURCE_URL
                     value: "jdbc:postgresql://postgres:5432/cameleer3?currentSchema=public"
-                  - name: OPENSEARCH_URL
-                    value: "http://opensearch:9200"
                   - name: CAMELEER_UI_ORIGIN
                     value: "http://192.168.50.86:30090"
   # UI ConfigMap: production API URL