feat: v2.0 Sprint 1 - Backup Verification & Retention Policy

- Add SHA-256 checksum generation for all backups
- Implement verify-backup command for integrity validation
- Add JSON metadata format (.meta.json) with full backup info
- Create retention policy engine with smart cleanup
- Add cleanup command with dry-run and pattern matching
- Integrate metadata generation into backup flow
- Maintain backward compatibility with legacy .info files

New commands:
- dbbackup verify-backup [files] - Verify backup integrity
- dbbackup cleanup [dir] - Clean old backups with retention policy

New packages:
- internal/metadata - Backup metadata management
- internal/verification - Checksum validation
- internal/retention - Retention policy engine

internal/metadata/metadata.go

@@ -0,0 +1,167 @@
+package metadata
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// BackupMetadata contains comprehensive information about a backup
+type BackupMetadata struct {
+	Version         string            `json:"version"`
+	Timestamp       time.Time         `json:"timestamp"`
+	Database        string            `json:"database"`
+	DatabaseType    string            `json:"database_type"`    // postgresql, mysql, mariadb
+	DatabaseVersion string            `json:"database_version"` // e.g., "PostgreSQL 15.3"
+	Host            string            `json:"host"`
+	Port            int               `json:"port"`
+	User            string            `json:"user"`
+	BackupFile      string            `json:"backup_file"`
+	SizeBytes       int64             `json:"size_bytes"`
+	SHA256          string            `json:"sha256"`
+	Compression     string            `json:"compression"` // none, gzip, pigz
+	BackupType      string            `json:"backup_type"` // full, incremental (for v2.0)
+	BaseBackup      string            `json:"base_backup,omitempty"`
+	Duration        float64           `json:"duration_seconds"`
+	ExtraInfo       map[string]string `json:"extra_info,omitempty"`
+}
+
+// ClusterMetadata contains metadata for cluster backups
+type ClusterMetadata struct {
+	Version      string             `json:"version"`
+	Timestamp    time.Time          `json:"timestamp"`
+	ClusterName  string             `json:"cluster_name"`
+	DatabaseType string             `json:"database_type"`
+	Host         string             `json:"host"`
+	Port         int                `json:"port"`
+	Databases    []BackupMetadata   `json:"databases"`
+	TotalSize    int64              `json:"total_size_bytes"`
+	Duration     float64            `json:"duration_seconds"`
+	ExtraInfo    map[string]string  `json:"extra_info,omitempty"`
+}
+
+// CalculateSHA256 computes the SHA-256 checksum of a file
+func CalculateSHA256(filePath string) (string, error) {
+	f, err := os.Open(filePath)
+	if err != nil {
+		return "", fmt.Errorf("failed to open file: %w", err)
+	}
+	defer f.Close()
+
+	hasher := sha256.New()
+	if _, err := io.Copy(hasher, f); err != nil {
+		return "", fmt.Errorf("failed to calculate checksum: %w", err)
+	}
+
+	return hex.EncodeToString(hasher.Sum(nil)), nil
+}
+
+// Save writes metadata to a .meta.json file
+func (m *BackupMetadata) Save() error {
+	metaPath := m.BackupFile + ".meta.json"
+	
+	data, err := json.MarshalIndent(m, "", "  ")
+	if err != nil {
+		return fmt.Errorf("failed to marshal metadata: %w", err)
+	}
+
+	if err := os.WriteFile(metaPath, data, 0644); err != nil {
+		return fmt.Errorf("failed to write metadata file: %w", err)
+	}
+
+	return nil
+}
+
+// Load reads metadata from a .meta.json file
+func Load(backupFile string) (*BackupMetadata, error) {
+	metaPath := backupFile + ".meta.json"
+	
+	data, err := os.ReadFile(metaPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read metadata file: %w", err)
+	}
+
+	var meta BackupMetadata
+	if err := json.Unmarshal(data, &meta); err != nil {
+		return nil, fmt.Errorf("failed to parse metadata: %w", err)
+	}
+
+	return &meta, nil
+}
+
+// SaveCluster writes cluster metadata to a .meta.json file
+func (m *ClusterMetadata) Save(targetFile string) error {
+	metaPath := targetFile + ".meta.json"
+	
+	data, err := json.MarshalIndent(m, "", "  ")
+	if err != nil {
+		return fmt.Errorf("failed to marshal cluster metadata: %w", err)
+	}
+
+	if err := os.WriteFile(metaPath, data, 0644); err != nil {
+		return fmt.Errorf("failed to write cluster metadata file: %w", err)
+	}
+
+	return nil
+}
+
+// LoadCluster reads cluster metadata from a .meta.json file
+func LoadCluster(targetFile string) (*ClusterMetadata, error) {
+	metaPath := targetFile + ".meta.json"
+	
+	data, err := os.ReadFile(metaPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read cluster metadata file: %w", err)
+	}
+
+	var meta ClusterMetadata
+	if err := json.Unmarshal(data, &meta); err != nil {
+		return nil, fmt.Errorf("failed to parse cluster metadata: %w", err)
+	}
+
+	return &meta, nil
+}
+
+// ListBackups scans a directory for backup files and returns their metadata
+func ListBackups(dir string) ([]*BackupMetadata, error) {
+	pattern := filepath.Join(dir, "*.meta.json")
+	matches, err := filepath.Glob(pattern)
+	if err != nil {
+		return nil, fmt.Errorf("failed to scan directory: %w", err)
+	}
+
+	var backups []*BackupMetadata
+	for _, metaFile := range matches {
+		// Extract backup file path (remove .meta.json suffix)
+		backupFile := metaFile[:len(metaFile)-len(".meta.json")]
+		
+		meta, err := Load(backupFile)
+		if err != nil {
+			// Skip invalid metadata files
+			continue
+		}
+		
+		backups = append(backups, meta)
+	}
+
+	return backups, nil
+}
+
+// FormatSize returns human-readable size
+func FormatSize(bytes int64) string {
+	const unit = 1024
+	if bytes < unit {
+		return fmt.Sprintf("%d B", bytes)
+	}
+	div, exp := int64(unit), 0
+	for n := bytes / unit; n >= unit; n /= unit {
+		div *= unit
+		exp++
+	}
+	return fmt.Sprintf("%.1f %ciB", float64(bytes)/float64(div), "KMGTPE"[exp])
+}