Add ZNC module

I'm running a ZNC server on my NixOS machine. I'd like to move it into a module, which uses systemd to start and manage it.

How does this module look? I'm curious to hear about my creating a user and group, and about the uid and gid for those. Are we really managing a central list of these ids? That list says that there's a max of ~300 ids. Is that right? This doesn't seem like a sustainable thing.
Why default = "root" ? I would have expected null so that it creates a znc user.