✨(backend) add django-treebeard to allow tree structure on documents

We choose to use Django-treebeard for its quality, performance and
stability. Adding tree structure to documents is as simple as
inheriting from the MP_Node class.

src/backend/pyproject.toml

@@ -38,6 +38,7 @@ dependencies = [
     "django-storages[s3]==1.14.4",
     "django-timezone-field>=5.1",
     "django==5.1.5",
+    "django-treebeard==4.7.1",
     "djangorestframework==3.15.2",
     "drf_spectacular==0.28.0",
     "dockerflow==2024.4.2",