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>

go.mod

@@ -7,20 +7,30 @@ require (
 	github.com/mark3labs/mcp-go v0.43.2
 	github.com/nats-io/nats.go v1.48.0
 	github.com/nats-io/nkeys v0.4.15
+	github.com/prometheus/client_golang v1.23.2
 	github.com/urfave/cli/v3 v3.6.2
 )
 
 require (
 	github.com/bahlo/generic-list-go v0.2.0 // indirect
+	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/buger/jsonparser v1.1.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/invopop/jsonschema v0.13.0 // indirect
 	github.com/klauspost/compress v1.18.0 // indirect
+	github.com/kylelemons/godebug v1.1.0 // indirect
 	github.com/mailru/easyjson v0.7.7 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/nats-io/nuid v1.0.1 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.66.1 // indirect
+	github.com/prometheus/procfs v0.16.1 // indirect
 	github.com/spf13/cast v1.7.1 // indirect
 	github.com/wk8/go-ordered-map/v2 v2.1.8 // indirect
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
+	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	golang.org/x/crypto v0.47.0 // indirect
 	golang.org/x/sys v0.40.0 // indirect
+	google.golang.org/protobuf v1.36.8 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )