neoprism: replace kanvibe with kannix
- Add kannix flake input (local git repo)
- 2configs/kannix.nix: kannix service on port 4243, nginx at kannix.lassul.us
- Git integration enabled (repos + worktrees in /var/lib/kannix/)
- SSH keys for kannix user
- Backup updated: /var/lib/kannix instead of /var/lib/kanvibe