feat: Phase 4 Task 6 - Restore decryption integration

- Added encryption flags to restore commands (--encryption-key-file, --encryption-key-env)
- Integrated DecryptBackupFile() into runRestoreSingle and runRestoreCluster
- Auto-detects encrypted backups via IsBackupEncrypted()
- Decrypts in-place before restore begins
- Tested: Encryption/decryption roundtrip validated successfully
- Phase 4 (AES-256-GCM encryption) now COMPLETE

All encryption features working:
✅ Backup encryption with --encrypt flag
✅ Restore decryption with --encryption-key-file flag
✅ Key loading from file or environment variable
✅ Metadata tracking (Encrypted bool, EncryptionAlgorithm)
✅ Roundtrip test passed: Original ≡ Decrypted

cmd/restore.go

@@ -10,6 +10,7 @@ import (
 	"syscall"
 	"time"
 
+	"dbbackup/internal/backup"
 	"dbbackup/internal/cloud"
 	"dbbackup/internal/database"
 	"dbbackup/internal/restore"
@@ -28,6 +29,10 @@ var (
 	restoreTarget    string
 	restoreVerbose   bool
 	restoreNoProgress bool
+	
+	// Encryption flags
+	restoreEncryptionKeyFile string
+	restoreEncryptionKeyEnv  string = "DBBACKUP_ENCRYPTION_KEY"
 )
 
 // restoreCmd represents the restore command
@@ -156,6 +161,8 @@ func init() {
 	restoreSingleCmd.Flags().StringVar(&restoreTarget, "target", "", "Target database name (defaults to original)")
 	restoreSingleCmd.Flags().BoolVar(&restoreVerbose, "verbose", false, "Show detailed restore progress")
 	restoreSingleCmd.Flags().BoolVar(&restoreNoProgress, "no-progress", false, "Disable progress indicators")
+	restoreSingleCmd.Flags().StringVar(&restoreEncryptionKeyFile, "encryption-key-file", "", "Path to encryption key file (required for encrypted backups)")
+	restoreSingleCmd.Flags().StringVar(&restoreEncryptionKeyEnv, "encryption-key-env", "DBBACKUP_ENCRYPTION_KEY", "Environment variable containing encryption key")
 
 	// Cluster restore flags
 	restoreClusterCmd.Flags().BoolVar(&restoreConfirm, "confirm", false, "Confirm and execute restore (required)")
@@ -164,6 +171,8 @@ func init() {
 	restoreClusterCmd.Flags().IntVar(&restoreJobs, "jobs", 0, "Number of parallel decompression jobs (0 = auto)")
 	restoreClusterCmd.Flags().BoolVar(&restoreVerbose, "verbose", false, "Show detailed restore progress")
 	restoreClusterCmd.Flags().BoolVar(&restoreNoProgress, "no-progress", false, "Disable progress indicators")
+	restoreClusterCmd.Flags().StringVar(&restoreEncryptionKeyFile, "encryption-key-file", "", "Path to encryption key file (required for encrypted backups)")
+	restoreClusterCmd.Flags().StringVar(&restoreEncryptionKeyEnv, "encryption-key-env", "DBBACKUP_ENCRYPTION_KEY", "Environment variable containing encryption key")
 }
 
 // runRestoreSingle restores a single database
@@ -214,6 +223,20 @@ func runRestoreSingle(cmd *cobra.Command, args []string) error {
 		}
 	}
 
+	// Check if backup is encrypted and decrypt if necessary
+	if backup.IsBackupEncrypted(archivePath) {
+		log.Info("Encrypted backup detected, decrypting...")
+		key, err := loadEncryptionKey(restoreEncryptionKeyFile, restoreEncryptionKeyEnv)
+		if err != nil {
+			return fmt.Errorf("encrypted backup requires encryption key: %w", err)
+		}
+		// Decrypt in-place (same path)
+		if err := backup.DecryptBackupFile(archivePath, archivePath, key, log); err != nil {
+			return fmt.Errorf("decryption failed: %w", err)
+		}
+		log.Info("Decryption completed successfully")
+	}
+
 	// Detect format
 	format := restore.DetectArchiveFormat(archivePath)
 	if format == restore.FormatUnknown {
@@ -340,6 +363,20 @@ func runRestoreCluster(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("archive not found: %s", archivePath)
 	}
 
+	// Check if backup is encrypted and decrypt if necessary
+	if backup.IsBackupEncrypted(archivePath) {
+		log.Info("Encrypted cluster backup detected, decrypting...")
+		key, err := loadEncryptionKey(restoreEncryptionKeyFile, restoreEncryptionKeyEnv)
+		if err != nil {
+			return fmt.Errorf("encrypted backup requires encryption key: %w", err)
+		}
+		// Decrypt in-place (same path)
+		if err := backup.DecryptBackupFile(archivePath, archivePath, key, log); err != nil {
+			return fmt.Errorf("decryption failed: %w", err)
+		}
+		log.Info("Cluster decryption completed successfully")
+	}
+
 	// Verify it's a cluster backup
 	format := restore.DetectArchiveFormat(archivePath)
 	if !format.IsClusterBackup() {