security: Implement HIGH priority security improvements

HIGH Priority Security Features:
- Path sanitization with filepath.Clean() for all user paths
- Path traversal attack prevention in backup/restore operations
- Secure config file permissions (0600 instead of 0644)
- SHA-256 checksum generation for all backup archives
- Checksum verification during restore operations
- Comprehensive audit logging for compliance

New Security Module (internal/security/):
- paths.go: ValidateBackupPath() and ValidateArchivePath()
- checksum.go: ChecksumFile(), VerifyChecksum(), LoadAndVerifyChecksum()
- audit.go: AuditLogger with structured event tracking

Integration Points:
- Backup engine: Path validation, checksum generation
- Restore engine: Path validation, checksum verification
- All backup/restore operations: Audit logging
- Configuration saves: Audit logging

Security Enhancements:
- .dbbackup.conf now created with 0600 permissions (owner-only)
- All archive files get .sha256 checksum files
- Restore warns if checksum verification fails but continues
- Audit events logged for all administrative operations
- User tracking via $USER/$USERNAME environment variables

Compliance Features:
- Audit trail for backups, restores, config changes
- Structured logging with timestamps, users, actions, results
- Event details include paths, sizes, durations, errors

Testing:
- All code compiles successfully
- Cross-platform build verified
- Ready for integration testing

internal/restore/engine.go

@@ -16,6 +16,7 @@ import (
 	"dbbackup/internal/database"
 	"dbbackup/internal/logger"
 	"dbbackup/internal/progress"
+	"dbbackup/internal/security"
 )
 
 // Engine handles database restore operations
@@ -101,12 +102,28 @@ func (la *loggerAdapter) Debug(msg string, args ...any) {
 func (e *Engine) RestoreSingle(ctx context.Context, archivePath, targetDB string, cleanFirst, createIfMissing bool) error {
 	operation := e.log.StartOperation("Single Database Restore")
 
+	// Validate and sanitize archive path
+	validArchivePath, pathErr := security.ValidateArchivePath(archivePath)
+	if pathErr != nil {
+		operation.Fail(fmt.Sprintf("Invalid archive path: %v", pathErr))
+		return fmt.Errorf("invalid archive path: %w", pathErr)
+	}
+	archivePath = validArchivePath
+
 	// Validate archive exists
 	if _, err := os.Stat(archivePath); os.IsNotExist(err) {
 		operation.Fail("Archive not found")
 		return fmt.Errorf("archive not found: %s", archivePath)
 	}
 
+	// Verify checksum if .sha256 file exists
+	if checksumErr := security.LoadAndVerifyChecksum(archivePath); checksumErr != nil {
+		e.log.Warn("Checksum verification failed", "error", checksumErr)
+		e.log.Warn("Continuing restore without checksum verification (use with caution)")
+	} else {
+		e.log.Info("✓ Archive checksum verified successfully")
+	}
+
 	// Detect archive format
 	format := DetectArchiveFormat(archivePath)
 	e.log.Info("Detected archive format", "format", format, "path", archivePath)
@@ -486,12 +503,28 @@ func (e *Engine) previewRestore(archivePath, targetDB string, format ArchiveForm
 func (e *Engine) RestoreCluster(ctx context.Context, archivePath string) error {
 	operation := e.log.StartOperation("Cluster Restore")
 
-	// Validate archive
+	// Validate and sanitize archive path
+	validArchivePath, pathErr := security.ValidateArchivePath(archivePath)
+	if pathErr != nil {
+		operation.Fail(fmt.Sprintf("Invalid archive path: %v", pathErr))
+		return fmt.Errorf("invalid archive path: %w", pathErr)
+	}
+	archivePath = validArchivePath
+
+	// Validate archive exists
 	if _, err := os.Stat(archivePath); os.IsNotExist(err) {
 		operation.Fail("Archive not found")
 		return fmt.Errorf("archive not found: %s", archivePath)
 	}
 
+	// Verify checksum if .sha256 file exists
+	if checksumErr := security.LoadAndVerifyChecksum(archivePath); checksumErr != nil {
+		e.log.Warn("Checksum verification failed", "error", checksumErr)
+		e.log.Warn("Continuing restore without checksum verification (use with caution)")
+	} else {
+		e.log.Info("✓ Cluster archive checksum verified successfully")
+	}
+
 	format := DetectArchiveFormat(archivePath)
 	if format != FormatClusterTarGz {
 		operation.Fail("Invalid cluster archive format")