This commit simplifies the `ExternalAPIs` type, which is now a pure
enumeration, moving the lookup of the Pubmed key at the usage sites.
This simplified the code quite a bit, and made some otherwise-effectful
functions pure.

This is safer than type Limit = Int

They are changed to MaybeFields ...Read instead when joining.

Remove joins, use optionalRestrict instead.

Also, remove cabal.project because stack.yaml and package.yaml cover
dependencies already.

Based on the suggestions in
https://kodimensional.dev/space-leak

Read in backend works now, insert and upsert not yet.