Also, there's no guarantee that the auth/refresh tokens won't collide, yet a constraint in the DB prevents them from colliding, so we might want to either have something that happens on `save()` retry a few times before giving up, or come up w/ a way of assigning secure tokens that are globally unique. Or we could remove the uniqueness constraint from the DB--I've heard that on very rare occasions, gmail's session IDs will collide and you'll temporarily have access to someone else's mailbox, but I'm not sure if that's an urban myth.