feat: add embedded systemd installer and Prometheus metrics

Systemd Integration:
- New 'dbbackup install' command creates service/timer units
- Supports single-database and cluster backup modes
- Automatic dbbackup user/group creation with proper permissions
- Hardened service units with security features
- Template units with configurable OnCalendar schedules
- 'dbbackup uninstall' for clean removal

Prometheus Metrics:
- 'dbbackup metrics export' for textfile collector format
- 'dbbackup metrics serve' runs HTTP exporter on port 9399
- Metrics: last_success_timestamp, rpo_seconds, backup_total, etc.
- Integration with node_exporter textfile collector
- --with-metrics flag during install

Technical:
- Systemd templates embedded with //go:embed
- Service units include ReadWritePaths, OOMScoreAdjust
- Metrics exporter caches with 30s TTL
- Graceful shutdown on SIGTERM

cmd/metrics.go

@@ -0,0 +1,138 @@
+package cmd
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"dbbackup/internal/prometheus"
+
+	"github.com/spf13/cobra"
+)
+
+var (
+	metricsInstance string
+	metricsOutput   string
+	metricsPort     int
+)
+
+// metricsCmd represents the metrics command
+var metricsCmd = &cobra.Command{
+	Use:   "metrics",
+	Short: "Prometheus metrics management",
+	Long: `Prometheus metrics management for dbbackup.
+
+Export metrics to a textfile for node_exporter, or run an HTTP server
+for direct Prometheus scraping.`,
+}
+
+// metricsExportCmd exports metrics to a textfile
+var metricsExportCmd = &cobra.Command{
+	Use:   "export",
+	Short: "Export metrics to textfile",
+	Long: `Export Prometheus metrics to a textfile for node_exporter.
+
+The textfile collector in node_exporter can scrape metrics from files
+in a designated directory (typically /var/lib/node_exporter/textfile_collector/).
+
+Examples:
+  # Export metrics to default location
+  dbbackup metrics export
+
+  # Export with custom output path
+  dbbackup metrics export --output /var/lib/dbbackup/metrics/dbbackup.prom
+
+  # Export for specific instance
+  dbbackup metrics export --instance production --output /var/lib/dbbackup/metrics/production.prom
+
+After export, configure node_exporter with:
+  --collector.textfile.directory=/var/lib/dbbackup/metrics/
+`,
+	RunE: func(cmd *cobra.Command, args []string) error {
+		return runMetricsExport(cmd.Context())
+	},
+}
+
+// metricsServeCmd runs the HTTP metrics server
+var metricsServeCmd = &cobra.Command{
+	Use:   "serve",
+	Short: "Run Prometheus HTTP server",
+	Long: `Run an HTTP server exposing Prometheus metrics.
+
+This starts a long-running daemon that serves metrics at /metrics.
+Prometheus can scrape this endpoint directly.
+
+Examples:
+  # Start server on default port 9399
+  dbbackup metrics serve
+
+  # Start server on custom port
+  dbbackup metrics serve --port 9100
+
+  # Run as systemd service (installed via 'dbbackup install --with-metrics')
+  sudo systemctl start dbbackup-exporter
+
+Endpoints:
+  /metrics  - Prometheus metrics
+  /health   - Health check (returns 200 OK)
+  /         - Service info page
+`,
+	RunE: func(cmd *cobra.Command, args []string) error {
+		return runMetricsServe(cmd.Context())
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(metricsCmd)
+	metricsCmd.AddCommand(metricsExportCmd)
+	metricsCmd.AddCommand(metricsServeCmd)
+
+	// Export flags
+	metricsExportCmd.Flags().StringVar(&metricsInstance, "instance", "default", "Instance name for metrics labels")
+	metricsExportCmd.Flags().StringVarP(&metricsOutput, "output", "o", "/var/lib/dbbackup/metrics/dbbackup.prom", "Output file path")
+
+	// Serve flags
+	metricsServeCmd.Flags().StringVar(&metricsInstance, "instance", "default", "Instance name for metrics labels")
+	metricsServeCmd.Flags().IntVarP(&metricsPort, "port", "p", 9399, "HTTP server port")
+}
+
+func runMetricsExport(ctx context.Context) error {
+	// Open catalog
+	cat, err := openCatalog()
+	if err != nil {
+		return fmt.Errorf("failed to open catalog: %w", err)
+	}
+	defer cat.Close()
+
+	// Create metrics writer
+	writer := prometheus.NewMetricsWriter(log, cat, metricsInstance)
+
+	// Write textfile
+	if err := writer.WriteTextfile(metricsOutput); err != nil {
+		return fmt.Errorf("failed to write metrics: %w", err)
+	}
+
+	log.Info("Exported metrics to textfile", "path", metricsOutput, "instance", metricsInstance)
+	return nil
+}
+
+func runMetricsServe(ctx context.Context) error {
+	// Setup signal handling
+	ctx, cancel := signal.NotifyContext(ctx, os.Interrupt, syscall.SIGTERM)
+	defer cancel()
+
+	// Open catalog
+	cat, err := openCatalog()
+	if err != nil {
+		return fmt.Errorf("failed to open catalog: %w", err)
+	}
+	defer cat.Close()
+
+	// Create exporter
+	exporter := prometheus.NewExporter(log, cat, metricsInstance, metricsPort)
+
+	// Run server (blocks until context is cancelled)
+	return exporter.Serve(ctx)
+}