Show last listing update time next to connection status in header

Add last_updated timestamp to /api/status endpoint by querying
MAX(last_seen) across both listing tables. Display it in the
HealthIndicator as relative time (e.g. "2h ago") with full
date/time in the tooltip on hover.

frontend/src/services/healthService.ts

@@ -6,6 +6,7 @@ export interface HealthCheckResult {
   status: HealthStatus;
   latencyMs?: number;
   error?: string;
+  lastUpdated?: string;
 }
 
 /**
@@ -39,6 +40,7 @@ export async function checkBackendHealth(): Promise<HealthCheckResult> {
       return {
         status: 'healthy',
         latencyMs,
+        lastUpdated: data.last_updated ?? undefined,
       };
     }