🐛(backend) move lock table closer to the insert operation targeted

We want to lock the table just before the insert we want to protect is
made. In the case of the perform_create action in the Document viewset,
an http call is made after the lock and can take a very long time,
blocking for nothing the table.

src/backend/core/api/viewsets.py

@@ -674,17 +674,8 @@ class DocumentViewSet(
 
         return drf.response.Response(serializer.data)
 
-    @transaction.atomic
     def perform_create(self, serializer):
         """Set the current user as creator and owner of the newly created object."""
-
-        # locks the table to ensure safe concurrent access
-        with connection.cursor() as cursor:
-            cursor.execute(
-                f'LOCK TABLE "{models.Document._meta.db_table}" '  # noqa: SLF001
-                "IN SHARE ROW EXCLUSIVE MODE;"
-            )
-
         # Remove file from validated_data as it's not a model field
         # Process it if present
         uploaded_file = serializer.validated_data.pop("file", None)
@@ -707,10 +698,18 @@ class DocumentViewSet(
                     {"file": ["Could not convert file content"]}
                 ) from err
 
-        obj = models.Document.add_root(
-            creator=self.request.user,
-            **serializer.validated_data,
-        )
+        with transaction.atomic():
+            # locks the table to ensure safe concurrent access
+            with connection.cursor() as cursor:
+                cursor.execute(
+                    f'LOCK TABLE "{models.Document._meta.db_table}" '  # noqa: SLF001
+                    "IN SHARE ROW EXCLUSIVE MODE;"
+                )
+
+            obj = models.Document.add_root(
+                creator=self.request.user,
+                **serializer.validated_data,
+            )
         serializer.instance = obj
         models.DocumentAccess.objects.create(
             document=obj,