fix: improve ClickHouse admin page, fix AgentHealth type error

Rewrite ClickHouse admin to show useful storage metrics instead of
often-empty system.events data. Add active queries section.

- Replace performance endpoint: query system.parts for disk size,
  uncompressed size, compression ratio, total rows, part count
- Add /queries endpoint querying system.processes for active queries
- Frontend: storage overview strip, tables with total size, active
  queries DataTable
- Fix AgentHealth.tsx type: agentId → instanceId in inline type cast

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -61,3 +61,8 @@
   color: var(--text-muted);
   font-family: var(--font-mono);
 }
+
+.queryText {
+  font-size: 0.75rem;
+  font-family: var(--font-mono);
+}