Fix GT job creation races (#7096)

It's possible to get more than 1 GT job in a task in the case of
concurrent creation requests.
This PR fixes this by introducing a task row-level lock in the DB for
the update transaction.

- Fixed GT job creation race condition

changelog.d/20231104_113802_mzhiltsov_fix_atomic_uses.md

@@ -0,0 +1,4 @@
+### Fixed
+
+- Data race condition during GT job creation
+  (<https://github.com/opencv/cvat/pull/7096>)

cvat/apps/engine/serializers.py

@@ -620,7 +620,7 @@ def to_representation(self, instance):
     @transaction.atomic
     def create(self, validated_data):
         task_id = validated_data.pop('task_id')
-        task = models.Task.objects.get(pk=task_id)
+        task = models.Task.objects.select_for_update().get(pk=task_id)
 
         if validated_data["type"] == models.JobType.GROUND_TRUTH:
             if not task.data: