Function set_credit_registration_config 

pub async fn set_credit_registration_config(
    conn: &mut PgConnection,
    course_module_id: Uuid,
    edit: &CourseModuleCreditRegistrationEdit,
) -> ModelResult<()>

Writes the module’s Suotar configuration row and reconciles its realisations. An unknown grade scale is refused here because otherwise it surfaces as no_grade_scale_mapping on every completion of the module, long after the teacher left the editor.