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.

Still, email is missing and the handling of user click.

However, the code doesn't work currently.