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-exporter.service

@@ -0,0 +1,37 @@
+[Unit]
+Description=DBBackup Prometheus Metrics Exporter
+Documentation=https://github.com/PlusOne/dbbackup
+After=network-online.target
+
+[Service]
+Type=simple
+User={{.User}}
+Group={{.Group}}
+
+# Security hardening
+NoNewPrivileges=yes
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+RestrictRealtime=yes
+LockPersonality=yes
+RemoveIPC=yes
+
+# Read-only access to catalog and backups
+ReadOnlyPaths=/var/lib/dbbackup
+
+# Network for HTTP server
+RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
+
+# Execution
+ExecStart={{.BinaryPath}} metrics serve --port {{.MetricsPort}} --config {{.ConfigPath}}
+ExecReload=/bin/kill -HUP $MAINPID
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target