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/verify.go

@@ -0,0 +1,141 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"dbbackup/internal/metadata"
+	"dbbackup/internal/verification"
+	"github.com/spf13/cobra"
+)
+
+var verifyBackupCmd = &cobra.Command{
+	Use:   "verify-backup [backup-file]",
+	Short: "Verify backup file integrity with checksums",
+	Long: `Verify the integrity of one or more backup files by comparing their SHA-256 checksums
+against the stored metadata. This ensures that backups have not been corrupted.
+
+Examples:
+  # Verify a single backup
+  dbbackup verify-backup /backups/mydb_20260115.dump
+
+  # Verify all backups in a directory
+  dbbackup verify-backup /backups/*.dump
+
+  # Quick verification (size check only, no checksum)
+  dbbackup verify-backup /backups/mydb.dump --quick
+
+  # Verify and show detailed information
+  dbbackup verify-backup /backups/mydb.dump --verbose`,
+	Args: cobra.MinimumNArgs(1),
+	RunE: runVerifyBackup,
+}
+
+var (
+	quickVerify   bool
+	verboseVerify bool
+)
+
+func init() {
+	rootCmd.AddCommand(verifyBackupCmd)
+	verifyBackupCmd.Flags().BoolVar(&quickVerify, "quick", false, "Quick verification (size check only)")
+	verifyBackupCmd.Flags().BoolVarP(&verboseVerify, "verbose", "v", false, "Show detailed information")
+}
+
+func runVerifyBackup(cmd *cobra.Command, args []string) error {
+	// Expand glob patterns
+	var backupFiles []string
+	for _, pattern := range args {
+		matches, err := filepath.Glob(pattern)
+		if err != nil {
+			return fmt.Errorf("invalid pattern %s: %w", pattern, err)
+		}
+		if len(matches) == 0 {
+			// Not a glob, use as-is
+			backupFiles = append(backupFiles, pattern)
+		} else {
+			backupFiles = append(backupFiles, matches...)
+		}
+	}
+
+	if len(backupFiles) == 0 {
+		return fmt.Errorf("no backup files found")
+	}
+
+	fmt.Printf("Verifying %d backup file(s)...\n\n", len(backupFiles))
+
+	successCount := 0
+	failureCount := 0
+
+	for _, backupFile := range backupFiles {
+		// Skip metadata files
+		if strings.HasSuffix(backupFile, ".meta.json") || 
+		   strings.HasSuffix(backupFile, ".sha256") || 
+		   strings.HasSuffix(backupFile, ".info") {
+			continue
+		}
+
+		fmt.Printf("📁 %s\n", filepath.Base(backupFile))
+
+		if quickVerify {
+			// Quick check: size only
+			err := verification.QuickCheck(backupFile)
+			if err != nil {
+				fmt.Printf("   ❌ FAILED: %v\n\n", err)
+				failureCount++
+				continue
+			}
+			fmt.Printf("   ✅ VALID (quick check)\n\n")
+			successCount++
+		} else {
+			// Full verification with SHA-256
+			result, err := verification.Verify(backupFile)
+			if err != nil {
+				return fmt.Errorf("verification error: %w", err)
+			}
+
+			if result.Valid {
+				fmt.Printf("   ✅ VALID\n")
+				if verboseVerify {
+					meta, _ := metadata.Load(backupFile)
+					fmt.Printf("   Size: %s\n", metadata.FormatSize(meta.SizeBytes))
+					fmt.Printf("   SHA-256: %s\n", meta.SHA256)
+					fmt.Printf("   Database: %s (%s)\n", meta.Database, meta.DatabaseType)
+					fmt.Printf("   Created: %s\n", meta.Timestamp.Format(time.RFC3339))
+				}
+				fmt.Println()
+				successCount++
+			} else {
+				fmt.Printf("   ❌ FAILED: %v\n", result.Error)
+				if verboseVerify {
+					if !result.FileExists {
+						fmt.Printf("   File does not exist\n")
+					} else if !result.MetadataExists {
+						fmt.Printf("   Metadata file missing\n")
+					} else if !result.SizeMatch {
+						fmt.Printf("   Size mismatch\n")
+					} else {
+						fmt.Printf("   Expected: %s\n", result.ExpectedSHA256)
+						fmt.Printf("   Got:      %s\n", result.CalculatedSHA256)
+					}
+				}
+				fmt.Println()
+				failureCount++
+			}
+		}
+	}
+
+	// Summary
+	fmt.Println(strings.Repeat("─", 50))
+	fmt.Printf("Total: %d backups\n", len(backupFiles))
+	fmt.Printf("✅ Valid: %d\n", successCount)
+	if failureCount > 0 {
+		fmt.Printf("❌ Failed: %d\n", failureCount)
+		os.Exit(1)
+	}
+
+	return nil
+}