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

cmd/cleanup.go

@@ -0,0 +1,152 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"dbbackup/internal/metadata"
+	"dbbackup/internal/retention"
+	"github.com/spf13/cobra"
+)
+
+var cleanupCmd = &cobra.Command{
+	Use:   "cleanup [backup-directory]",
+	Short: "Clean up old backups based on retention policy",
+	Long: `Remove old backup files based on retention policy while maintaining minimum backup count.
+
+The retention policy ensures:
+1. Backups older than --retention-days are eligible for deletion
+2. At least --min-backups most recent backups are always kept
+3. Both conditions must be met for deletion
+
+Examples:
+  # Clean up backups older than 30 days (keep at least 5)
+  dbbackup cleanup /backups --retention-days 30 --min-backups 5
+
+  # Dry run to see what would be deleted
+  dbbackup cleanup /backups --retention-days 7 --dry-run
+
+  # Clean up specific database backups only
+  dbbackup cleanup /backups --pattern "mydb_*.dump"
+
+  # Aggressive cleanup (keep only 3 most recent)
+  dbbackup cleanup /backups --retention-days 1 --min-backups 3`,
+	Args: cobra.ExactArgs(1),
+	RunE: runCleanup,
+}
+
+var (
+	retentionDays int
+	minBackups    int
+	dryRun        bool
+	cleanupPattern string
+)
+
+func init() {
+	rootCmd.AddCommand(cleanupCmd)
+	cleanupCmd.Flags().IntVar(&retentionDays, "retention-days", 30, "Delete backups older than this many days")
+	cleanupCmd.Flags().IntVar(&minBackups, "min-backups", 5, "Always keep at least this many backups")
+	cleanupCmd.Flags().BoolVar(&dryRun, "dry-run", false, "Show what would be deleted without actually deleting")
+	cleanupCmd.Flags().StringVar(&cleanupPattern, "pattern", "", "Only clean up backups matching this pattern (e.g., 'mydb_*.dump')")
+}
+
+func runCleanup(cmd *cobra.Command, args []string) error {
+	backupDir := args[0]
+
+	// Validate directory exists
+	if !dirExists(backupDir) {
+		return fmt.Errorf("backup directory does not exist: %s", backupDir)
+	}
+
+	// Create retention policy
+	policy := retention.Policy{
+		RetentionDays: retentionDays,
+		MinBackups:    minBackups,
+		DryRun:        dryRun,
+	}
+
+	fmt.Printf("🗑️  Cleanup Policy:\n")
+	fmt.Printf("   Directory: %s\n", backupDir)
+	fmt.Printf("   Retention: %d days\n", policy.RetentionDays)
+	fmt.Printf("   Min backups: %d\n", policy.MinBackups)
+	if cleanupPattern != "" {
+		fmt.Printf("   Pattern: %s\n", cleanupPattern)
+	}
+	if dryRun {
+		fmt.Printf("   Mode: DRY RUN (no files will be deleted)\n")
+	}
+	fmt.Println()
+
+	var result *retention.CleanupResult
+	var err error
+
+	// Apply policy
+	if cleanupPattern != "" {
+		result, err = retention.CleanupByPattern(backupDir, cleanupPattern, policy)
+	} else {
+		result, err = retention.ApplyPolicy(backupDir, policy)
+	}
+
+	if err != nil {
+		return fmt.Errorf("cleanup failed: %w", err)
+	}
+
+	// Display results
+	fmt.Printf("📊 Results:\n")
+	fmt.Printf("   Total backups: %d\n", result.TotalBackups)
+	fmt.Printf("   Eligible for deletion: %d\n", result.EligibleForDeletion)
+	
+	if len(result.Deleted) > 0 {
+		fmt.Printf("\n")
+		if dryRun {
+			fmt.Printf("🔍 Would delete %d backup(s):\n", len(result.Deleted))
+		} else {
+			fmt.Printf("✅ Deleted %d backup(s):\n", len(result.Deleted))
+		}
+		for _, file := range result.Deleted {
+			fmt.Printf("   - %s\n", filepath.Base(file))
+		}
+	}
+
+	if len(result.Kept) > 0 && len(result.Kept) <= 10 {
+		fmt.Printf("\n📦 Kept %d backup(s):\n", len(result.Kept))
+		for _, file := range result.Kept {
+			fmt.Printf("   - %s\n", filepath.Base(file))
+		}
+	} else if len(result.Kept) > 10 {
+		fmt.Printf("\n📦 Kept %d backup(s)\n", len(result.Kept))
+	}
+
+	if !dryRun && result.SpaceFreed > 0 {
+		fmt.Printf("\n💾 Space freed: %s\n", metadata.FormatSize(result.SpaceFreed))
+	}
+
+	if len(result.Errors) > 0 {
+		fmt.Printf("\n⚠️  Errors:\n")
+		for _, err := range result.Errors {
+			fmt.Printf("   - %v\n", err)
+		}
+	}
+
+	fmt.Println(strings.Repeat("─", 50))
+	
+	if dryRun {
+		fmt.Println("✅ Dry run completed (no files were deleted)")
+	} else if len(result.Deleted) > 0 {
+		fmt.Println("✅ Cleanup completed successfully")
+	} else {
+		fmt.Println("ℹ️  No backups eligible for deletion")
+	}
+
+	return nil
+}
+
+func dirExists(path string) bool {
+	info, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+	return info.IsDir()
+}