feat: add vendor tenant metrics dashboard

Fleet overview page at /vendor/metrics showing per-tenant operational
metrics (agents, CPU, heap, HTTP requests, ingestion drops, uptime).
Queries each tenant's server via the new POST /api/v1/admin/server-metrics/query
REST API instead of direct ClickHouse access, supporting future per-tenant
CH instances.

Backend: TenantMetricsService fires 11 metric queries per tenant
concurrently over a 5-minute window, assembles into a summary snapshot.
ServerApiClient.queryServerMetrics() handles the M2M authenticated POST.

Frontend: VendorMetricsPage with KPI strip (fleet totals) and per-tenant
table with color-coded badges and heap usage bars. Auto-refreshes every 60s.

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

ui/src/components/Layout.tsx

@@ -109,6 +109,14 @@ export function Layout() {
           >
             Certificates
           </div>
+          <div
+            style={{ padding: '6px 12px 6px 36px', fontSize: 13, cursor: 'pointer',
+              fontWeight: isActive(location, '/vendor/metrics') ? 600 : 400,
+              color: isActive(location, '/vendor/metrics') ? 'var(--amber)' : 'var(--text-muted)' }}
+            onClick={() => navigate('/vendor/metrics')}
+          >
+            Metrics
+          </div>
           <div
             style={{ padding: '6px 12px 6px 36px', fontSize: 13, cursor: 'pointer',
               fontWeight: isActive(location, '/vendor/infrastructure') ? 600 : 400,