feat: add Prometheus metrics to listener service

Add an optional Prometheus metrics HTTP endpoint to the listener for
monitoring deployment operations. Includes four metrics:

- homelab_deploy_deployments_total (counter with status/action/error_code)
- homelab_deploy_deployment_duration_seconds (histogram with action/success)
- homelab_deploy_deployment_in_progress (gauge)
- homelab_deploy_info (gauge with hostname/tier/role/version)

New CLI flags: --metrics-enabled, --metrics-addr (default :9972)
New NixOS options: metrics.enable, metrics.address, metrics.openFirewall

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -26,7 +26,7 @@
             pname = "homelab-deploy";
             inherit version;
             src = ./.;
-            vendorHash = "sha256-JXa+obN62zrrwXlplqojY7dvEunUqDdSTee6N8c5JTg=";
+            vendorHash = "sha256-CN+l0JbQu+HDfotkt3PUFzBexHCHpCKIIZpAQRyojBk=";
             subPackages = [ "cmd/homelab-deploy" ];
           };
           default = self.packages.${system}.homelab-deploy;