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>

docker-compose.yml

@@ -10,18 +10,5 @@ services:
     volumes:
       - pgdata:/home/postgres/pgdata/data
 
-  opensearch:
-    image: opensearchproject/opensearch:2.19.0
-    ports:
-      - "9200:9200"
-      - "9300:9300"
-    environment:
-      discovery.type: single-node
-      DISABLE_SECURITY_PLUGIN: "true"
-      OPENSEARCH_JAVA_OPTS: "-Xms512m -Xmx512m"
-    volumes:
-      - osdata:/usr/share/opensearch/data
-
 volumes:
   pgdata:
-  osdata: