model: rename ID => RID, implement sql Valuer

As in "Resource" ID. I also changed the base32 alphabet to lowercase,
because I subjectively think it looks nicer.

Run modernize

See https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/modernize

It's mostly interface{} -> any, but also two quite useful applications
of slices.Contains.

gofmt

model: introduce ID type

Based on UUIDv7 but represented to users in a more compact base32 form,
though all of that are internal implementation details.