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.tsx

@@ -0,0 +1,65 @@
+import { StatCard, DataTable, ProgressBar } from '@cameleer/design-system';
+import type { Column } from '@cameleer/design-system';
+import { useClickHouseStatus, useClickHouseTables, useClickHousePerformance, useIndexerPipeline } from '../../api/queries/admin/clickhouse';
+import styles from './ClickHouseAdminPage.module.css';
+
+export default function ClickHouseAdminPage() {
+  const { data: status, isError: statusError } = useClickHouseStatus();
+  const { data: tables } = useClickHouseTables();
+  const { data: perf } = useClickHousePerformance();
+  const { data: pipeline } = useIndexerPipeline();
+  const unreachable = statusError || (status && !status.reachable);
+
+  const tableColumns: Column<any>[] = [
+    { key: 'name', header: 'Table', sortable: true },
+    { key: 'engine', header: 'Engine' },
+    { key: 'rowCount', header: 'Rows', sortable: true, render: (v) => Number(v).toLocaleString() },
+    { key: 'dataSize', header: 'Size', sortable: true },
+    { key: 'partitionCount', header: 'Partitions', sortable: true },
+  ];
+
+  return (
+    <div>
+      <div className={styles.statStrip}>
+        <StatCard label="Status" value={unreachable ? 'Disconnected' : status ? 'Connected' : '\u2014'} accent={unreachable ? 'error' : status ? 'success' : undefined} />
+        <StatCard label="Version" value={status?.version ?? '\u2014'} />
+        <StatCard label="Uptime" value={status?.uptime ?? '\u2014'} />
+      </div>
+
+      {pipeline && (
+        <div className={styles.pipelineCard}>
+          <div className={styles.pipelineTitle}>Indexer Pipeline</div>
+          <ProgressBar value={pipeline.maxQueueSize > 0 ? (pipeline.queueDepth / pipeline.maxQueueSize) * 100 : 0} />
+          <div className={styles.pipelineMetrics}>
+            <span>Queue: {pipeline.queueDepth}/{pipeline.maxQueueSize}</span>
+            <span>Indexed: {pipeline.indexedCount.toLocaleString()}</span>
+            <span>Failed: {pipeline.failedCount}</span>
+            <span>Rate: {pipeline.indexingRate.toFixed(1)}/s</span>
+          </div>
+        </div>
+      )}
+
+      {perf && (
+        <div className={styles.statStrip}>
+          <StatCard label="Select Queries" value={perf.queryCount.toLocaleString()} />
+          <StatCard label="Insert Queries" value={perf.insertQueryCount.toLocaleString()} />
+          <StatCard label="Memory" value={perf.memoryUsage} />
+          <StatCard label="Rows Read" value={perf.readRows.toLocaleString()} />
+        </div>
+      )}
+
+      <div className={styles.tableSection}>
+        <div className={styles.tableHeader}>
+          <span className={styles.tableTitle}>Tables ({(tables || []).length})</span>
+        </div>
+        <DataTable
+          columns={tableColumns}
+          data={(tables || []).map((t: any) => ({ ...t, id: t.name }))}
+          sortable
+          pageSize={20}
+          flush
+        />
+      </div>
+    </div>
+  );
+}