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/pom.xml

@@ -47,16 +47,6 @@
             <groupId>org.flywaydb</groupId>
             <artifactId>flyway-database-postgresql</artifactId>
         </dependency>
-        <dependency>
-            <groupId>org.opensearch.client</groupId>
-            <artifactId>opensearch-java</artifactId>
-            <version>2.19.0</version>
-        </dependency>
-        <dependency>
-            <groupId>org.opensearch.client</groupId>
-            <artifactId>opensearch-rest-client</artifactId>
-            <version>2.19.0</version>
-        </dependency>
         <dependency>
             <groupId>com.clickhouse</groupId>
             <artifactId>clickhouse-jdbc</artifactId>
@@ -126,12 +116,6 @@
             <artifactId>testcontainers-junit-jupiter</artifactId>
             <scope>test</scope>
         </dependency>
-        <dependency>
-            <groupId>org.opensearch</groupId>
-            <artifactId>opensearch-testcontainers</artifactId>
-            <version>2.1.1</version>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.testcontainers</groupId>
             <artifactId>testcontainers-clickhouse</artifactId>