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>

ui/src/pages/Admin/ClickHouseAdminPage.module.css

@@ -0,0 +1,63 @@
+.statStrip {
+  display: flex;
+  gap: 10px;
+  margin-bottom: 16px;
+  flex-wrap: wrap;
+}
+
+.pipelineCard {
+  background: var(--bg-surface);
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-lg);
+  box-shadow: var(--shadow-card);
+  padding: 16px 20px;
+  margin-bottom: 16px;
+}
+
+.pipelineTitle {
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--text-primary);
+  margin-bottom: 8px;
+}
+
+.pipelineMetrics {
+  display: flex;
+  gap: 24px;
+  margin-top: 8px;
+  font-size: 12px;
+  color: var(--text-muted);
+}
+
+.pipelineMetrics span {
+  font-family: var(--font-mono);
+}
+
+.tableSection {
+  background: var(--bg-surface);
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-lg);
+  box-shadow: var(--shadow-card);
+  margin-bottom: 16px;
+  overflow: hidden;
+}
+
+.tableHeader {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 12px 16px;
+  border-bottom: 1px solid var(--border-subtle);
+}
+
+.tableTitle {
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--text-primary);
+}
+
+.tableMeta {
+  font-size: 11px;
+  color: var(--text-muted);
+  font-family: var(--font-mono);
+}