refactor(ssh): consolidate sshd enable and port 22 into modules/ssh.nix

The daemon enable and the firewall port were duplicated in each sshd host
(T400, Mac Pro, RPi5). Move both into modules/ssh.nix so importing it both
hardens and enables sshd; drop the per-host copies. No build change: the
three hosts evaluate to identical derivations.

Closes #51

modules/ssh.nix

@@ -1,8 +1,11 @@
-# Key-only sshd hardening, imported by hosts that run sshd (T400, Mac Pro,
-# RPi5). Authorized keys are owned per-user by the registry (modules/users.nix),
-# not here.
+# sshd for the hosts that run it (T400, Mac Pro, RPi5): enable the daemon, open
+# port 22, and apply a key-only policy. Authorized keys are owned per-user by the
+# registry (modules/users.nix), not here.
 { ... }:
 {
+  services.openssh.enable = true;
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
   services.openssh.settings = {
     PasswordAuthentication = false; # keys only
     KbdInteractiveAuthentication = false; # no keyboard-interactive fallback