feat: Phase 4 Tasks 3-4 - CLI encryption integration

Integrated encryption into backup workflow:

cmd/encryption.go:
- loadEncryptionKey() - loads from file or env var
- Supports base64-encoded keys (32 bytes)
- Supports raw 32-byte keys
- Supports passphrases (PBKDF2 derivation)
- Priority: --encryption-key-file > DBBACKUP_ENCRYPTION_KEY

cmd/backup_impl.go:
- encryptLatestBackup() - finds and encrypts single backups
- encryptLatestClusterBackup() - encrypts cluster backups
- findLatestBackup() - locates most recent backup file
- findLatestClusterBackup() - locates cluster backup
- Encryption applied after successful backup
- Integrated into all backup modes (cluster, single, sample)

internal/backup/encryption.go:
- EncryptBackupFile() - encrypts backup in-place
- DecryptBackupFile() - decrypts to new file
- IsBackupEncrypted() - checks metadata/file format
- Updates .meta.json with encryption info
- Replaces original with encrypted version

internal/metadata/metadata.go:
- Added Encrypted bool field
- Added EncryptionAlgorithm string field
- Tracks encryption status in backup metadata

internal/metadata/save.go:
- Helper to save BackupMetadata to .meta.json

tests/encryption_smoke_test.sh:
- Basic smoke test for encryption/decryption
- Verifies data integrity
- Tests with env var key source

CLI Flags (already existed):
--encrypt                      Enable encryption
--encryption-key-file PATH     Key file path
--encryption-key-env VAR       Env var name (default: DBBACKUP_ENCRYPTION_KEY)

Usage Examples:
  # Encrypt with key file
  ./dbbackup backup single mydb --encrypt --encryption-key-file /path/to/key

  # Encrypt with env var
  export DBBACKUP_ENCRYPTION_KEY="base64_encoded_key"
  ./dbbackup backup single mydb --encrypt

  # Cluster backup with encryption
  ./dbbackup backup cluster --encrypt --encryption-key-file key.txt

Features:
✅ Post-backup encryption (doesn't slow down backup itself)
✅ In-place encryption (overwrites original)
✅ Metadata tracking (encrypted flag)
✅ Multiple key sources (file/env/passphrase)
✅ Base64 and raw key support
✅ PBKDF2 for passphrases
✅ Automatic latest backup detection
✅ Works with all backup modes

Status: ENCRYPTION FULLY INTEGRATED ✅
Next: Task 5 - Restore decryption integration

internal/backup/encryption.go

@@ -0,0 +1,114 @@
+package backup
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"dbbackup/internal/crypto"
+	"dbbackup/internal/logger"
+	"dbbackup/internal/metadata"
+)
+
+// EncryptBackupFile encrypts a backup file in-place
+// The original file is replaced with the encrypted version
+func EncryptBackupFile(backupPath string, key []byte, log logger.Logger) error {
+	log.Info("Encrypting backup file", "file", filepath.Base(backupPath))
+	
+	// Validate key
+	if err := crypto.ValidateKey(key); err != nil {
+		return fmt.Errorf("invalid encryption key: %w", err)
+	}
+
+	// Create encryptor
+	encryptor := crypto.NewAESEncryptor()
+
+	// Generate encrypted file path
+	encryptedPath := backupPath + ".encrypted.tmp"
+
+	// Encrypt file
+	if err := encryptor.EncryptFile(backupPath, encryptedPath, key); err != nil {
+		// Clean up temp file on failure
+		os.Remove(encryptedPath)
+		return fmt.Errorf("encryption failed: %w", err)
+	}
+
+	// Update metadata to indicate encryption
+	metaPath := backupPath + ".meta.json"
+	if _, err := os.Stat(metaPath); err == nil {
+		// Load existing metadata
+		meta, err := metadata.Load(metaPath)
+		if err != nil {
+			log.Warn("Failed to load metadata for encryption update", "error", err)
+		} else {
+			// Mark as encrypted
+			meta.Encrypted = true
+			meta.EncryptionAlgorithm = string(crypto.AlgorithmAES256GCM)
+
+			// Save updated metadata
+			if err := metadata.Save(metaPath, meta); err != nil {
+				log.Warn("Failed to update metadata with encryption info", "error", err)
+			}
+		}
+	}
+
+	// Remove original unencrypted file
+	if err := os.Remove(backupPath); err != nil {
+		log.Warn("Failed to remove original unencrypted file", "error", err)
+		// Don't fail - encrypted file exists
+	}
+
+	// Rename encrypted file to original name
+	if err := os.Rename(encryptedPath, backupPath); err != nil {
+		return fmt.Errorf("failed to rename encrypted file: %w", err)
+	}
+
+	log.Info("Backup encrypted successfully", "file", filepath.Base(backupPath))
+	return nil
+}
+
+// IsBackupEncrypted checks if a backup file is encrypted
+func IsBackupEncrypted(backupPath string) bool {
+	// Check metadata first
+	metaPath := backupPath + ".meta.json"
+	if meta, err := metadata.Load(metaPath); err == nil {
+		return meta.Encrypted
+	}
+	
+	// Fallback: check if file starts with encryption nonce
+	file, err := os.Open(backupPath)
+	if err != nil {
+		return false
+	}
+	defer file.Close()
+	
+	// Try to read nonce - if it succeeds, likely encrypted
+	nonce := make([]byte, crypto.NonceSize)
+	if n, err := file.Read(nonce); err != nil || n != crypto.NonceSize {
+		return false
+	}
+	
+	return true
+}
+
+// DecryptBackupFile decrypts an encrypted backup file
+// Creates a new decrypted file
+func DecryptBackupFile(encryptedPath, outputPath string, key []byte, log logger.Logger) error {
+	log.Info("Decrypting backup file", "file", filepath.Base(encryptedPath))
+
+	// Validate key
+	if err := crypto.ValidateKey(key); err != nil {
+		return fmt.Errorf("invalid decryption key: %w", err)
+	}
+
+	// Create encryptor
+	encryptor := crypto.NewAESEncryptor()
+
+	// Decrypt file
+	if err := encryptor.DecryptFile(encryptedPath, outputPath, key); err != nil {
+		return fmt.Errorf("decryption failed (wrong key?): %w", err)
+	}
+
+	log.Info("Backup decrypted successfully", "output", filepath.Base(outputPath))
+	return nil
+}