✨(backend) add API endpoint to move a document in the document tree

Only administrators or owners of a document can move it to a target
document for which they are also administrator or owner.

We allow different moving modes:
- first-child: move the document as the first child of the target
- last-child: move the document as the last child of the target
- first-sibling: move the document as the first sibling of the target
- last-sibling: move the document as the last sibling of the target
- left: move the document as sibling ordered just before the target
- right: move the document as sibling ordered just after the target

The whole subtree below the document that is being moved, moves as
well and remains below the document after it is moved.

src/backend/core/api/viewsets.py

@@ -311,6 +311,7 @@ class DocumentMetadata(drf.metadata.SimpleMetadata):
         return simple_metadata
 
 
+# pylint: disable=too-many-public-methods
 class DocumentViewSet(
     drf.mixins.CreateModelMixin,
     drf.mixins.DestroyModelMixin,
@@ -490,6 +491,65 @@ class DocumentViewSet(
             {"id": str(document.id)}, status=status.HTTP_201_CREATED
         )
 
+    @drf.decorators.action(detail=True, methods=["post"])
+    @transaction.atomic
+    def move(self, request, *args, **kwargs):
+        """
+        Move a document to another location within the document tree.
+
+        The user must be an administrator or owner of both the document being moved
+        and the target parent document.
+        """
+        user = request.user
+        document = self.get_object()  # including permission checks
+
+        # Validate the input payload
+        serializer = serializers.MoveDocumentSerializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+        validated_data = serializer.validated_data
+
+        target_document_id = validated_data["target_document_id"]
+        try:
+            target_document = models.Document.objects.get(
+                id=target_document_id, ancestors_deleted_at__isnull=True
+            )
+        except models.Document.DoesNotExist:
+            return drf.response.Response(
+                {"target_document_id": "Target parent document does not exist."},
+                status=status.HTTP_400_BAD_REQUEST,
+            )
+
+        position = validated_data["position"]
+        message = None
+
+        if position in [
+            enums.MoveNodePositionChoices.FIRST_CHILD,
+            enums.MoveNodePositionChoices.LAST_CHILD,
+        ]:
+            if not target_document.get_abilities(user).get("move"):
+                message = (
+                    "You do not have permission to move documents "
+                    "as a child to this target document."
+                )
+        elif not target_document.is_root():
+            if not target_document.get_parent().get_abilities(user).get("move"):
+                message = (
+                    "You do not have permission to move documents "
+                    "as a sibling of this target document."
+                )
+
+        if message:
+            return drf.response.Response(
+                {"target_document_id": message},
+                status=status.HTTP_400_BAD_REQUEST,
+            )
+
+        document.move(target_document, pos=position)
+
+        return drf.response.Response(
+            {"message": "Document moved successfully."}, status=status.HTTP_200_OK
+        )
+
     @drf.decorators.action(
         detail=True,
         methods=["get", "post"],