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/api/vendor-hooks.ts

@@ -1,6 +1,6 @@
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { api } from './client';
-import type { VendorTenantSummary, VendorTenantDetail, CreateTenantRequest, TenantResponse, LicenseResponse, AuditLogPage, AuditLogFilters } from '../types/api';
+import type { VendorTenantSummary, VendorTenantDetail, CreateTenantRequest, TenantResponse, LicenseResponse, AuditLogPage, AuditLogFilters, TenantMetricsEntry } from '../types/api';
 
 export function useVendorTenants() {
   return useQuery<VendorTenantSummary[]>({
@@ -179,3 +179,13 @@ export function useInfraChDetail(tenantId: string) {
     enabled: !!tenantId,
   });
 }
+
+// --- Tenant Metrics ---
+
+export function useTenantMetrics() {
+  return useQuery<TenantMetricsEntry[]>({
+    queryKey: ['vendor', 'metrics'],
+    queryFn: () => api.get('/vendor/metrics'),
+    refetchInterval: 60_000,
+  });
+}