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>

README.md

@@ -61,6 +61,8 @@ homelab-deploy listener \
 | `--timeout` | No | Deployment timeout in seconds (default: 600) |
 | `--deploy-subject` | No | NATS subjects to subscribe to (repeatable) |
 | `--discover-subject` | No | Discovery subject (default: `deploy.discover`) |
+| `--metrics-enabled` | No | Enable Prometheus metrics endpoint |
+| `--metrics-addr` | No | Metrics HTTP server address (default: `:9972`) |
 
 #### Subject Templates
 
@@ -209,6 +211,9 @@ Add the module to your NixOS configuration:
 | `deploySubjects` | list of string | see below | Subjects to subscribe to |
 | `discoverSubject` | string | `"deploy.discover"` | Discovery subject |
 | `environment` | attrs | `{}` | Additional environment variables |
+| `metrics.enable` | bool | `false` | Enable Prometheus metrics endpoint |
+| `metrics.address` | string | `":9972"` | Metrics HTTP server address |
+| `metrics.openFirewall` | bool | `false` | Open firewall for metrics port |
 
 Default `deploySubjects`:
 ```nix
@@ -219,6 +224,80 @@ Default `deploySubjects`:
 ]
 ```
 
+## Prometheus Metrics
+
+The listener can expose Prometheus metrics for monitoring deployment operations.
+
+### Enabling Metrics
+
+**CLI:**
+```bash
+homelab-deploy listener \
+  --hostname myhost \
+  --tier prod \
+  --nats-url nats://nats.example.com:4222 \
+  --nkey-file /run/secrets/listener.nkey \
+  --flake-url git+https://git.example.com/user/nixos-configs.git \
+  --metrics-enabled \
+  --metrics-addr :9972
+```
+
+**NixOS module:**
+```nix
+services.homelab-deploy.listener = {
+  enable = true;
+  tier = "prod";
+  natsUrl = "nats://nats.example.com:4222";
+  nkeyFile = "/run/secrets/homelab-deploy-nkey";
+  flakeUrl = "git+https://git.example.com/user/nixos-configs.git";
+  metrics = {
+    enable = true;
+    address = ":9972";
+    openFirewall = true;  # Optional: open firewall for Prometheus scraping
+  };
+};
+```
+
+### Available Metrics
+
+| Metric | Type | Labels | Description |
+|--------|------|--------|-------------|
+| `homelab_deploy_deployments_total` | Counter | `status`, `action`, `error_code` | Total deployment requests processed |
+| `homelab_deploy_deployment_duration_seconds` | Histogram | `action`, `success` | Deployment execution time |
+| `homelab_deploy_deployment_in_progress` | Gauge | - | 1 if deployment running, 0 otherwise |
+| `homelab_deploy_info` | Gauge | `hostname`, `tier`, `role`, `version` | Static instance metadata |
+
+**Label values:**
+- `status`: `completed`, `failed`, `rejected`
+- `action`: `switch`, `boot`, `test`, `dry-activate`
+- `error_code`: `invalid_action`, `invalid_revision`, `already_running`, `build_failed`, `timeout`, or empty
+- `success`: `true`, `false`
+
+### HTTP Endpoints
+
+| Endpoint | Description |
+|----------|-------------|
+| `/metrics` | Prometheus metrics in text format |
+| `/health` | Health check (returns `ok`) |
+
+### Example Prometheus Queries
+
+```promql
+# Average deployment duration (last hour)
+rate(homelab_deploy_deployment_duration_seconds_sum[1h]) /
+rate(homelab_deploy_deployment_duration_seconds_count[1h])
+
+# Deployment success rate (last 24 hours)
+sum(rate(homelab_deploy_deployments_total{status="completed"}[24h])) /
+sum(rate(homelab_deploy_deployments_total{status=~"completed|failed"}[24h]))
+
+# 95th percentile deployment time
+histogram_quantile(0.95, rate(homelab_deploy_deployment_duration_seconds_bucket[1h]))
+
+# Currently running deployments across all hosts
+sum(homelab_deploy_deployment_in_progress)
+```
+
 ## Message Protocol
 
 ### Deploy Request