feat(rpi5): add host configuration (boot, network, sshd)

Tie the RPi5 submodules together: import hardware-config, docker.nix and
reverse-proxy.nix; pin networking.hostName to the flake attr name so nh
resolves; use U-Boot/extlinux boot (raspberry-pi-5 profile supplies kernel +
firmware); enable key-only sshd and a default-deny firewall opening 22.
Headless -- swaywm.nix is not imported, so swayDesktop stays off.

system/machine/RPi5/configuration.nix

@@ -0,0 +1,40 @@
+# Raspberry Pi 5 (aarch64) headless server. Two roles, split into submodules:
+# ./docker.nix (Docker host with a network socket) and ./reverse-proxy.nix
+# (native nginx). The raspberry-pi-5 nixos-hardware profile (kernel, firmware,
+# device tree) and key-only sshd (../../modules/ssh.nix) are layered on in the
+# flake host table. Install notes: see ./README.md.
+{ ... }:
+{
+  imports = [
+    ./hardware-configuration.nix
+    ./docker.nix
+    ./reverse-proxy.nix
+  ];
+
+  # Match the flake's nixosConfigurations attribute name so `nh os switch`
+  # (which selects by the local hostname) resolves without an explicit -H flag.
+  networking.hostName = "lyrathorpe-rpi5";
+
+  # Headless server: the Sway desktop is intentionally not set up. swaywm.nix is
+  # not imported and features.swayDesktop.enable defaults to false (declared in
+  # system/modules/features.nix), so this host keeps plain TTY/SSH login.
+
+  # Raspberry Pi boots via U-Boot + extlinux, not GRUB/systemd-boot. The
+  # raspberry-pi-5 nixos-hardware profile supplies the kernel, firmware and
+  # device tree.
+  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;
+
+  # 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.
+  networking.firewall.enable = true;
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
+  # See `man configuration.nix` / the stateVersion docs before changing.
+  system.stateVersion = "26.05";
+}