Add system alerts log with nav badge and Admin Alerts tab.

Persist job and ingestion warnings/errors for 7 days, surface a dismissible top-nav badge, treat stale OHLCV as a warning (e.g. ticker renames), and show market bar age on the ticker freshness chip.

frontend/src/hooks/useAdmin.ts

@@ -337,6 +337,41 @@ export function useToggleJob() {
   });
 }
 
+// ── System events ──
+
+export function useSystemEvents(days = 7) {
+  return useQuery({
+    queryKey: ['admin', 'system-events', days],
+    queryFn: () => adminApi.listSystemEvents({ days }),
+    refetchInterval: 30_000,
+  });
+}
+
+export function useSystemEventSummary(days = 7) {
+  return useQuery({
+    queryKey: ['admin', 'system-events-summary', days],
+    queryFn: () => adminApi.getSystemEventSummary(days),
+    refetchInterval: 30_000,
+  });
+}
+
+export function useAcknowledgeSystemEvents() {
+  const qc = useQueryClient();
+  const { addToast } = useToast();
+
+  return useMutation({
+    mutationFn: (days = 7) => adminApi.acknowledgeSystemEvents(days),
+    onSuccess: (data) => {
+      qc.invalidateQueries({ queryKey: ['admin', 'system-events'] });
+      qc.invalidateQueries({ queryKey: ['admin', 'system-events-summary'] });
+      addToast('success', `Dismissed ${data.acknowledged} alert${data.acknowledged === 1 ? '' : 's'}`);
+    },
+    onError: (error: Error) => {
+      addToast('error', error.message || 'Failed to dismiss alerts');
+    },
+  });
+}
+
 export function useTriggerJob() {
   const qc = useQueryClient();
   const { addToast } = useToast();