feat: add durable schedule management and multi-node crash recovery

internal/storage/mysql/task_store.go

@@ -354,7 +354,7 @@ func (s *TaskStore) CancelTask(ctx context.Context, token auth.Token, id core.ID
 		if err := tx.QueryRowContext(ctx, "SELECT owner_kind FROM pulse_tasks WHERE namespace_id=? AND id=?", token.NamespaceID, id).Scan(&owner); err != nil {
 			return err
 		}
-		if owner.Valid {
+		if owner.Valid && owner.String != "schedule" {
 			return fault.Conflict
 		}
 		now, err := dbNow(ctx, tx)