🐛(backend) use computed_link_reach in handle_onboarding_document

In the model method User::_handle_onboarding_documents_access we do not
allow using documents with restricted link_reach to be added as
onboarding documents. To check the real link_reach of the document, we
must use instead the computed_link_reach to be sure that a sub document
can also be used and compute its correct link_reach.

src/backend/core/models.py

@@ -244,7 +244,7 @@ class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
                     )
                     continue
 
-                if document.link_reach == LinkReachChoices.RESTRICTED:
+                if document.computed_link_reach == LinkReachChoices.RESTRICTED:
                     logger.warning(
                         "Onboarding on a restricted document is not allowed. Must be public or "
                         "connected. Restricted document: %s",