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

cmd/root.go

@@ -6,12 +6,14 @@ import (
 
 	"dbbackup/internal/config"
 	"dbbackup/internal/logger"
+	"dbbackup/internal/security"
 	"github.com/spf13/cobra"
 )
 
 var (
-	cfg *config.Config
-	log logger.Logger
+	cfg         *config.Config
+	log         logger.Logger
+	auditLogger *security.AuditLogger
 )
 
 // rootCmd represents the base command when called without any subcommands
@@ -57,6 +59,9 @@ For help with specific commands, use: dbbackup [command] --help`,
 func Execute(ctx context.Context, config *config.Config, logger logger.Logger) error {
 	cfg = config
 	log = logger
+	
+	// Initialize audit logger
+	auditLogger = security.NewAuditLogger(logger, true)
 
 	// Set version info
 	rootCmd.Version = fmt.Sprintf("%s (built: %s, commit: %s)",