feat: host heartbeat status and dashboard metrics (2.2)

Track agent.heartbeat and report.daily.ssh per host; show online/stale
on Hosts UI; dashboard counters and SAC_HEARTBEAT_STALE_MINUTES config.

frontend/src/api.ts

@@ -76,6 +76,9 @@ export interface HostSummary {
   ipv4: string | null;
   last_seen_at: string;
   event_count: number | null;
+  last_heartbeat_at: string | null;
+  last_daily_report_at: string | null;
+  agent_status: "online" | "stale" | "unknown";
 }
 
 export interface HostListResponse {
@@ -116,6 +119,9 @@ export async function resolveProblem(problemId: number): Promise<{ id: number; s
 export interface DashboardSummary {
   events_last_24h: number;
   hosts_total: number;
+  hosts_stale: number;
+  heartbeats_24h: number;
+  daily_reports_24h: number;
   problems_open: number;
   severity_24h: Record<string, number>;
   recent_events: EventSummary[];