This has just been absent for too long! We've lost a lot of data about
when poses were first modeled, which is a shame.
But I want this in now, because I was just doing caching on
/rainbow-pool.json, and realized that _labeling_ poses is another way
pet states can update rather than just being created!
So we need an `updated_at` field, to be able to quickly detect edits
that require us to invalidate the cache on
`PetState.all_supported_poses`. I'll do that next!
