feat: support durable task cancellation without foreign keys

internal/transport/httpserver/tasks.go

@@ -78,7 +78,11 @@ func registerTasks(mux *http.ServeMux, s *service.Service) {
 			return nil, 0, err
 		}
 		out, err := s.CancelTask(r.Context(), t, core.ID(strings.TrimSuffix(command, ":cancel")), key, in)
-		return out, 200, err
+		status := 200
+		if out.Task.State == pulse.TaskRunning {
+			status = 202
+		}
+		return out, status, err
 	}))
 	mux.HandleFunc("POST /v1/queues", wrap(func(r *http.Request, t auth.Token) (any, int, error) {
 		var in service.QueueInput