♻️(backend) simplify roles by returning only the max role

We were returning the list of roles a user has on a document (direct
and inherited). Now that we introduced priority on roles, we are able
to determine what is the max role and return only this one.

This commit also changes the role that is returned for the restricted
reach: we now return None because the role is not relevant in this
case.

Author: sampaccoud

src/backend/core/tests/documents/test_api_documents_trashbin.py

@@ -74,7 +74,6 @@ def test_api_documents_trashbin_format():
             "accesses_view": True,
             "ai_transform": True,
             "ai_translate": True,
-            "ancestors_links_definitions": {},
             "attachment_upload": True,
             "children_create": True,
             "children_list": True,