Add NixOS module for payjoin-mailroom

[spacebear21]

Expose a nixosModules.payjoin-mailroom flake output that provides a systemd service. Operators declare settings as native Nix attrsets (serialized to TOML), keep secrets in an environment file, and get DynamicUser, state persistence, and auto-start on boot out of the box.

Disclosure: co-authored by Claude Code
Another disclosure: I don't run nixOS so can't validate that this works as intended

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[benalleng]

TACK adcbab0 built with a minimal flake on a test nixos machine.

# /etc/nixos/flake.nix
{                                                                                 
  description = "Payjoin-mailroom test flake";                                         
                                                                                       
  inputs = {                                                                      
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";                             
    rust-payjoin = {                                                              
      url = "github:spacebear21/rust-payjoin/payjoin-mailroom-nixos-module";
    };                                                                            
  };                                                                              
                                                                                       
  outputs = { self, nixpkgs, rust-payjoin, ... }: {                               
   nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {                 
     modules = [                                                                 
         rust-payjoin.nixosModules.payjoin-mailroom                                
        ./configuration.nix                                                       
      ];                                                                          
    };                                                                            
  };                                                                              
}

I did not add any flags or specific configs. I only confirmed that I could see the payjojn-mailroom as enabled in the systemctl