add delta sync before each reconcile, have it not repeat if it's been done in the last minute

sync.go

@@ -44,3 +44,20 @@ func (s *Server) SyncRoster(ctx context.Context) error {
 	log.Printf("synced %d users (since=%d)", len(users), since)
 	return s.store.SetLastSync(started)
 }
+
+// syncIfStale runs a delta sync unless one completed within maxAge (1 min)
+// Serialized so a burst of runs triggers one sync, not one each.
+func (s *Server) syncIfStale(ctx context.Context, maxAge time.Duration) error {
+	s.syncMu.Lock()
+	defer s.syncMu.Unlock()
+
+	last, err := s.store.LastSync()
+	if err != nil {
+		return err
+	}
+	if time.Since(time.Unix(last, 0)) < maxAge {
+		return nil
+	}
+	return s.SyncRoster(ctx)
+}
+