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

internal/server/http/catchup.go

@@ -89,8 +89,16 @@ func (s *Server) runCatchup(ctx context.Context, hostID string, now time.Time) {
 	if host.AlwaysOn {
 		return // mode flipped during settle window
 	}
-	if host.CurrentJobID != nil {
-		return // a job is already running; don't pile on
+	// Skip if a backup is already queued or running for this host —
+	// don't pile a catch-up on top of in-flight work. (hosts.current_job_id
+	// is not maintained, so we check the jobs table directly.)
+	active, err := s.deps.Store.HasActiveBackupJob(ctx, hostID)
+	if err != nil {
+		slog.Warn("catchup: check active backup", "host_id", hostID, "err", err)
+		return
+	}
+	if active {
+		return
 	}
 	schedules, err := s.deps.Store.ListSchedulesByHost(ctx, hostID)
 	if err != nil {