♻️(backend) stop requiring owner for non-root documents

If root documents are guaranteed to have a owner, non-root documents
will automatically have them as owner by inheritance. We should not
require non-root documents to have their own direct owner because
this will make it difficult to manage access rights when we move
documents around or when we want to remove access rights for someone
on a document subtree... There should be as few overrides as possible.

src/backend/core/models.py

@@ -1121,9 +1121,12 @@ class DocumentAccess(BaseAccess):
         is_owner_or_admin = role in PRIVILEGED_ROLES
 
         if self.role == RoleChoices.OWNER:
-            can_delete = (
-                role == RoleChoices.OWNER
-                and DocumentAccess.objects.filter(
+            can_delete = role == RoleChoices.OWNER and (
+                # check if document is not root trying to avoid an extra query
+                # "document_path" is annotated by the viewset's list method
+                len(getattr(self, "document_path", "")) > Document.steplen
+                or not self.document.is_root()
+                or DocumentAccess.objects.filter(
                     document_id=self.document_id, role=RoleChoices.OWNER
                 ).count()
                 > 1