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>

cameleer3-server-app/src/main/resources/application.yml

@@ -37,20 +37,15 @@ ingestion:
   batch-size: 5000
   flush-interval-ms: 1000
 
-opensearch:
-  url: ${OPENSEARCH_URL:http://localhost:9200}
-  index-prefix: ${CAMELEER_OPENSEARCH_INDEX_PREFIX:executions-}
-  queue-size: ${CAMELEER_OPENSEARCH_QUEUE_SIZE:10000}
-  debounce-ms: ${CAMELEER_OPENSEARCH_DEBOUNCE_MS:2000}
-  log-index-prefix: ${CAMELEER_LOG_INDEX_PREFIX:logs-}
-  log-retention-days: ${CAMELEER_LOG_RETENTION_DAYS:7}
-
 cameleer:
   body-size-limit: ${CAMELEER_BODY_SIZE_LIMIT:16384}
+  indexer:
+    debounce-ms: ${CAMELEER_INDEXER_DEBOUNCE_MS:2000}
+    queue-size: ${CAMELEER_INDEXER_QUEUE_SIZE:10000}
   retention-days: ${CAMELEER_RETENTION_DAYS:30}
   storage:
     metrics: ${CAMELEER_STORAGE_METRICS:postgres}
-    search: ${CAMELEER_STORAGE_SEARCH:opensearch}
+    search: ${CAMELEER_STORAGE_SEARCH:clickhouse}
     stats: ${CAMELEER_STORAGE_STATS:clickhouse}
     diagrams: ${CAMELEER_STORAGE_DIAGRAMS:clickhouse}
     events: ${CAMELEER_STORAGE_EVENTS:clickhouse}