Convert worker to Fly cronjobs

[shaldengeki]

Right now our worker is a persistent process in its own app. This is relatively expensive; we get billed as long as the instance is up.

I think ideally we'd use something like https://github.com/fly-apps/cron-manager and have all our workers (which are really just cronjobs) be scheduled by a central app. That should reduce our bills.