Function update_modules 

pub async fn update_modules(
    conn: &mut PgConnection,
    course_id: Uuid,
    updates: ModuleUpdates,
) -> ModelResult<()>

Applies the module editor’s changes to course_id in one transaction.

Rejects the whole update if any referenced module or chapter is not in the course, or if a completion registration link fails validate_completion_registration_link.