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/prometheus/textfile.go

@@ -0,0 +1,245 @@
+// Package prometheus provides Prometheus metrics for dbbackup
+package prometheus
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+
+	"dbbackup/internal/catalog"
+	"dbbackup/internal/logger"
+)
+
+// MetricsWriter writes metrics in Prometheus text format
+type MetricsWriter struct {
+	log      logger.Logger
+	catalog  catalog.Catalog
+	instance string
+}
+
+// NewMetricsWriter creates a new MetricsWriter
+func NewMetricsWriter(log logger.Logger, cat catalog.Catalog, instance string) *MetricsWriter {
+	return &MetricsWriter{
+		log:      log,
+		catalog:  cat,
+		instance: instance,
+	}
+}
+
+// BackupMetrics holds metrics for a single database
+type BackupMetrics struct {
+	Database      string
+	Engine        string
+	LastSuccess   time.Time
+	LastDuration  time.Duration
+	LastSize      int64
+	TotalBackups  int
+	SuccessCount  int
+	FailureCount  int
+	Verified      bool
+	RPOSeconds    float64
+}
+
+// WriteTextfile writes metrics to a Prometheus textfile collector file
+func (m *MetricsWriter) WriteTextfile(path string) error {
+	metrics, err := m.collectMetrics()
+	if err != nil {
+		return fmt.Errorf("failed to collect metrics: %w", err)
+	}
+
+	output := m.formatMetrics(metrics)
+
+	// Atomic write: write to temp file, then rename
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return fmt.Errorf("failed to create directory %s: %w", dir, err)
+	}
+
+	tmpPath := path + ".tmp"
+	if err := os.WriteFile(tmpPath, []byte(output), 0644); err != nil {
+		return fmt.Errorf("failed to write temp file: %w", err)
+	}
+
+	if err := os.Rename(tmpPath, path); err != nil {
+		os.Remove(tmpPath)
+		return fmt.Errorf("failed to rename temp file: %w", err)
+	}
+
+	m.log.Debug("Wrote metrics to textfile", "path", path, "databases", len(metrics))
+	return nil
+}
+
+// collectMetrics gathers metrics from the catalog
+func (m *MetricsWriter) collectMetrics() ([]BackupMetrics, error) {
+	if m.catalog == nil {
+		return nil, fmt.Errorf("catalog not available")
+	}
+
+	ctx := context.Background()
+
+	// Get recent backups using Search with limit
+	query := &catalog.SearchQuery{
+		Limit: 1000,
+	}
+	entries, err := m.catalog.Search(ctx, query)
+	if err != nil {
+		return nil, fmt.Errorf("failed to search backups: %w", err)
+	}
+
+	// Group by database
+	byDB := make(map[string]*BackupMetrics)
+
+	for _, e := range entries {
+		key := e.Database
+		if key == "" {
+			key = "unknown"
+		}
+
+		metrics, ok := byDB[key]
+		if !ok {
+			metrics = &BackupMetrics{
+				Database: key,
+				Engine:   e.DatabaseType,
+			}
+			byDB[key] = metrics
+		}
+
+		metrics.TotalBackups++
+
+		isSuccess := e.Status == catalog.StatusCompleted || e.Status == catalog.StatusVerified
+		if isSuccess {
+			metrics.SuccessCount++
+			// Track most recent success
+			if e.CreatedAt.After(metrics.LastSuccess) {
+				metrics.LastSuccess = e.CreatedAt
+				metrics.LastDuration = time.Duration(e.Duration * float64(time.Second))
+				metrics.LastSize = e.SizeBytes
+				metrics.Verified = e.VerifiedAt != nil && e.VerifyValid != nil && *e.VerifyValid
+				metrics.Engine = e.DatabaseType
+			}
+		} else {
+			metrics.FailureCount++
+		}
+	}
+
+	// Calculate RPO for each database
+	now := time.Now()
+	for _, metrics := range byDB {
+		if !metrics.LastSuccess.IsZero() {
+			metrics.RPOSeconds = now.Sub(metrics.LastSuccess).Seconds()
+		}
+	}
+
+	// Convert to slice and sort
+	result := make([]BackupMetrics, 0, len(byDB))
+	for _, metrics := range byDB {
+		result = append(result, *metrics)
+	}
+	sort.Slice(result, func(i, j int) bool {
+		return result[i].Database < result[j].Database
+	})
+
+	return result, nil
+}
+
+// formatMetrics formats metrics in Prometheus exposition format
+func (m *MetricsWriter) formatMetrics(metrics []BackupMetrics) string {
+	var b strings.Builder
+
+	// Timestamp of metrics generation
+	now := time.Now().Unix()
+
+	// Header comment
+	b.WriteString("# DBBackup Prometheus Metrics\n")
+	b.WriteString(fmt.Sprintf("# Generated at: %s\n", time.Now().Format(time.RFC3339)))
+	b.WriteString(fmt.Sprintf("# Instance: %s\n", m.instance))
+	b.WriteString("\n")
+
+	// dbbackup_last_success_timestamp
+	b.WriteString("# HELP dbbackup_last_success_timestamp Unix timestamp of last successful backup\n")
+	b.WriteString("# TYPE dbbackup_last_success_timestamp gauge\n")
+	for _, met := range metrics {
+		if !met.LastSuccess.IsZero() {
+			b.WriteString(fmt.Sprintf("dbbackup_last_success_timestamp{instance=%q,database=%q,engine=%q} %d\n",
+				m.instance, met.Database, met.Engine, met.LastSuccess.Unix()))
+		}
+	}
+	b.WriteString("\n")
+
+	// dbbackup_last_backup_duration_seconds
+	b.WriteString("# HELP dbbackup_last_backup_duration_seconds Duration of last successful backup in seconds\n")
+	b.WriteString("# TYPE dbbackup_last_backup_duration_seconds gauge\n")
+	for _, met := range metrics {
+		if met.LastDuration > 0 {
+			b.WriteString(fmt.Sprintf("dbbackup_last_backup_duration_seconds{instance=%q,database=%q,engine=%q} %.2f\n",
+				m.instance, met.Database, met.Engine, met.LastDuration.Seconds()))
+		}
+	}
+	b.WriteString("\n")
+
+	// dbbackup_last_backup_size_bytes
+	b.WriteString("# HELP dbbackup_last_backup_size_bytes Size of last successful backup in bytes\n")
+	b.WriteString("# TYPE dbbackup_last_backup_size_bytes gauge\n")
+	for _, met := range metrics {
+		if met.LastSize > 0 {
+			b.WriteString(fmt.Sprintf("dbbackup_last_backup_size_bytes{instance=%q,database=%q,engine=%q} %d\n",
+				m.instance, met.Database, met.Engine, met.LastSize))
+		}
+	}
+	b.WriteString("\n")
+
+	// dbbackup_backup_total (counter)
+	b.WriteString("# HELP dbbackup_backup_total Total number of backup attempts\n")
+	b.WriteString("# TYPE dbbackup_backup_total counter\n")
+	for _, met := range metrics {
+		b.WriteString(fmt.Sprintf("dbbackup_backup_total{instance=%q,database=%q,status=\"success\"} %d\n",
+			m.instance, met.Database, met.SuccessCount))
+		b.WriteString(fmt.Sprintf("dbbackup_backup_total{instance=%q,database=%q,status=\"failure\"} %d\n",
+			m.instance, met.Database, met.FailureCount))
+	}
+	b.WriteString("\n")
+
+	// dbbackup_rpo_seconds
+	b.WriteString("# HELP dbbackup_rpo_seconds Recovery Point Objective - seconds since last successful backup\n")
+	b.WriteString("# TYPE dbbackup_rpo_seconds gauge\n")
+	for _, met := range metrics {
+		if met.RPOSeconds > 0 {
+			b.WriteString(fmt.Sprintf("dbbackup_rpo_seconds{instance=%q,database=%q} %.0f\n",
+				m.instance, met.Database, met.RPOSeconds))
+		}
+	}
+	b.WriteString("\n")
+
+	// dbbackup_backup_verified
+	b.WriteString("# HELP dbbackup_backup_verified Whether the last backup was verified (1=yes, 0=no)\n")
+	b.WriteString("# TYPE dbbackup_backup_verified gauge\n")
+	for _, met := range metrics {
+		verified := 0
+		if met.Verified {
+			verified = 1
+		}
+		b.WriteString(fmt.Sprintf("dbbackup_backup_verified{instance=%q,database=%q} %d\n",
+			m.instance, met.Database, verified))
+	}
+	b.WriteString("\n")
+
+	// dbbackup_scrape_timestamp
+	b.WriteString("# HELP dbbackup_scrape_timestamp Unix timestamp when metrics were collected\n")
+	b.WriteString("# TYPE dbbackup_scrape_timestamp gauge\n")
+	b.WriteString(fmt.Sprintf("dbbackup_scrape_timestamp{instance=%q} %d\n", m.instance, now))
+
+	return b.String()
+}
+
+// GenerateMetricsString returns metrics as a string (for HTTP endpoint)
+func (m *MetricsWriter) GenerateMetricsString() (string, error) {
+	metrics, err := m.collectMetrics()
+	if err != nil {
+		return "", err
+	}
+	return m.formatMetrics(metrics), nil
+}