Add declarative SNM mail stack on mx1 with DNS-01, DANE, MTA-STS

mx1 runs Simple NixOS Mailserver (Postfix/Dovecot/Rspamd/OpenDKIM) for
cnx.email. The TLS cert is obtained via ACME DNS-01 using a dedicated,
scoped TSIG key (acme_mx1) that ns1 authorizes for only
_acme-challenge.mx1 and _acme-challenge.mta-sts on the cnx.email zone, so
the credential can write nothing else. Mailbox passwords are auto-minted
by a clan vars generator (four-word passphrase + number).

DANE TLSA (3 1 1) is published for _25._tcp.mx1; --reuse-key keeps the
key digest stable across renewals. MTA-STS is enforced via a Caddy vhost
serving the policy on :443 from the same cert (mta-sts SAN). Firewall
opens 25/587/465/143/993/443; 80 stays closed.

flake.nix

@@ -3,6 +3,9 @@
   inputs.nixpkgs.follows = "clan-core/nixpkgs";
   inputs.treefmt-nix.url = "github:numtide/treefmt-nix";
   inputs.treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
+  # Simple NixOS Mailserver, pinned to the branch matching clan-core's nixpkgs.
+  inputs.nixos-mailserver.url = "gitlab:simple-nixos-mailserver/nixos-mailserver/nixos-25.11";
+  inputs.nixos-mailserver.inputs.nixpkgs.follows = "nixpkgs";
 
   outputs =
     {