feat(monitor): save state changes for group monitors

checkGroup() now detects status transitions and persists them as
state change records, same as individual monitors. Groups also
get StatusChangedAt updates and log entries for down/recovery
events. This enables history and SLA views for group monitors.

internal/monitor/checks.go

@@ -239,10 +239,24 @@ func (e *Engine) checkGroup(_ context.Context, site models.Site) {
 		status = models.StatusPending
 	}
 
+	var prev models.Status
 	e.applyState(site.ID, func(s *models.Site) {
+		prev = s.Status
 		s.Status = status
+		if status != prev && prev != models.StatusPending {
+			s.StatusChangedAt = time.Now()
+		}
 	})
 	e.recordCheck(site.ID, 0, !status.IsBroken())
+
+	if status != prev && prev != models.StatusPending {
+		e.enqueueWrite(writeStateChange{siteID: site.ID, fromStatus: string(prev), toStatus: string(status)})
+		if status.IsBroken() {
+			e.AddLog(fmt.Sprintf("Group '%s' is %s", site.Name, status))
+		} else if prev.IsBroken() {
+			e.AddLog(fmt.Sprintf("Group '%s' recovered", site.Name))
+		}
+	}
 }
 
 func (e *Engine) EnqueueProbeCheck(siteID int, nodeID string, latencyNs int64, isUp bool) {