fix(catchup): guard on real in-flight backup check; add scheduler tests

internal/store/jobs.go

@@ -270,6 +270,22 @@ func (s *Store) LatestJobByKind(ctx context.Context, hostID, kind string) (*Job,
 	return &j, nil
 }
 
+// HasActiveBackupJob reports whether the host has a backup job that is
+// still queued or running. The catch-up scheduler uses this to avoid
+// dispatching a duplicate backup alongside one already in flight
+// (hosts.current_job_id is not maintained, so this is the authoritative
+// in-flight check).
+func (s *Store) HasActiveBackupJob(ctx context.Context, hostID string) (bool, error) {
+	var exists bool
+	err := s.db.QueryRowContext(ctx,
+		`SELECT EXISTS(SELECT 1 FROM jobs WHERE host_id = ? AND kind = 'backup' AND status IN ('queued','running'))`,
+		hostID).Scan(&exists)
+	if err != nil {
+		return false, fmt.Errorf("store: has active backup job: %w", err)
+	}
+	return exists, nil
+}
+
 // HasJobOfKind reports whether any job of the given kind exists for
 // this host, regardless of status. Used by the auto-init path on
 // agent hello to decide whether to dispatch a fresh `restic init` —