mutate_object does not handle OneToOneFields

The `mutate_object` method of the `LettercraftMutation` (for handling mutation requests to the backend) has separate handlers for relational fields. The method decides on the handler by checking `field.many_to_many` and `field.many_to_one`:

https://github.com/CentreForDigitalHumanities/lettercraft/blob/dd3fb51ef2bd3f5eb8f2745608d4b340c98c49c5/backend/graphql_app/LettercraftMutation.py#L110-L115

This does not account for the [OneToOneField](https://docs.djangoproject.com/en/5.1/ref/models/fields/#onetoonefield) - this is also a relational field, but it is not `many_to_many` or `many_to_one`, so it won't be handled properly and the mutation will return an error.

A OneToOneField is essentially a ForeignKey with a uniqueness requirement. I *think* you could just use the handler for ForeignKey fields here. In that case, you could just adjust this tree as follows:

```py
if field.many_to_many is True:
many_to_many_fields.append(key)
elif field.is_relation is True:
one_to_many_fields.append(key)
else:
simple_fields.append(key)
```

I didn't test this yet, however.

Start in backend/graphql_app/LettercraftMutation.py around lines 110-115 and inspect how mutate_object classifies relational fields. Reproduce a mutation involving a Django OneToOneField, then confirm it is handled without an error and that existing many-to-many, foreign-key, and simple-field behavior remains intact.

Written by the indexing model from the issue text.