feat: add early fk_field configuration validation

[tnware]

Summary

• validate reverse relation config early during form setup
• raise ImproperlyConfigured when fk_field does not exist on the reverse model
• raise ImproperlyConfigured when fk_field is not a ForeignKey or OneToOneField
• raise ImproperlyConfigured when fk_field points to a different parent model than the admin model
• add edge-case tests for all three invalid configuration scenarios

Validation

• uv run python manage.py test

Closes #3