Function apply_email_update 

async fn apply_email_update(
    tx: &mut PgConnection,
    user_id: Uuid,
    new_email: &str,
) -> ModelResult<()>

Writes the new email onto user_details and keeps users.email_domain in step, within an already-open transaction. The clear_email_verification trigger drops proof of the old address as part of the user_details update.