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

internal/installer/templates/dbbackup-cluster.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Database Cluster Backup Timer
+Documentation=https://github.com/PlusOne/dbbackup
+
+[Timer]
+OnCalendar={{.Schedule}}
+Persistent=true
+RandomizedDelaySec=1800
+
+[Install]
+WantedBy=timers.target