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

hosts/RPi5/configuration.nix

@@ -25,15 +25,12 @@
   boot.loader.grub.enable = false;
   boot.loader.generic-extlinux-compatible.enable = true;
 
-  # Remote administration. Key-only policy and the authorized key come from
-  # ../../modules/ssh.nix; here we just enable the daemon and open the port.
-  services.openssh.enable = true;
+  # Remote administration: the daemon, port 22 and key-only policy all come from
+  # ../../modules/ssh.nix.
 
-  # Default-deny inbound. Open only SSH here; the Docker and nginx submodules
-  # open their own ports (Docker via a source-restricted nftables rule, nginx
-  # via 80/443). List-valued, so these merge with the submodule definitions.
+  # Default-deny inbound; the Docker and nginx submodules open their own ports
+  # (Docker via a source-restricted nftables rule, nginx via 80/443).
   networking.firewall.enable = true;
-  networking.firewall.allowedTCPPorts = [ 22 ];
 
   # See `man configuration.nix` / the stateVersion docs before changing.
   system.stateVersion = "26.05";