feat: option to use Fly.io as deployment platform

See #7 for reasoning

Start by reading issue #7 for the stated reasoning and requirements around deployment platforms. Then inspect the repository's current deployment setup to determine what an option for Fly.io would need to cover. Done means the project can be deployed through the requested Fly.io option and the behavior is documented or tested.

索引モデルが issue の本文から書いたものです。