fix: secure admin endpoints with authentication middleware (issue #4)

- Add config parameter to AuthService constructor
- Implement proper config-based auth checks in middleware
- Wrap all admin endpoints (users, groups, tokens) with authentication
- Apply granular scopes: admin:users:*, admin:groups:*, admin:tokens:*
- Maintain backward compatibility when config is nil

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

auth/auth.go

@@ -26,13 +26,15 @@ type AuthContext struct {
 type AuthService struct {
 	db     *badger.DB
 	logger *logrus.Logger
+	config *types.Config
 }
 
 // NewAuthService creates a new authentication service
-func NewAuthService(db *badger.DB, logger *logrus.Logger) *AuthService {
+func NewAuthService(db *badger.DB, logger *logrus.Logger, config *types.Config) *AuthService {
 	return &AuthService{
 		db:     db,
 		logger: logger,
+		config: config,
 	}
 }