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/types/api.ts

@@ -155,3 +155,48 @@ export interface AuditLogFilters {
   page?: number;
   size?: number;
 }
+
+// Tenant metrics (from server /api/v1/admin/metrics/summary)
+export interface AgentMetrics {
+  live: number;
+  stale: number;
+  dead: number;
+  shutdown: number;
+}
+
+export interface IngestionMetrics {
+  bufferDepth: number;
+  dropsPerMinute: number;
+}
+
+export interface ServerMetrics {
+  cpuUsage: number;
+  heapUsedBytes: number;
+  heapMaxBytes: number;
+  uptimeSeconds: number;
+  threadCount: number;
+}
+
+export interface HttpMetrics {
+  requestsPerMinute: number;
+  errorRate: number;
+}
+
+export interface MetricsSummary {
+  collectedAt: string;
+  agents: AgentMetrics;
+  ingestion: IngestionMetrics;
+  server: ServerMetrics;
+  http: HttpMetrics;
+  authFailuresPerMinute: number;
+}
+
+export interface TenantMetricsEntry {
+  tenantId: string;
+  tenantName: string;
+  slug: string;
+  tier: string;
+  status: string;
+  serverState: string;
+  metrics: MetricsSummary | null;
+}