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>

server/server.go

@@ -118,7 +118,7 @@ func NewServer(config *types.Config) (*Server, error) {
 	server.revisionService = storage.NewRevisionService(storageService)
 
 	// Initialize authentication service
-	server.authService = auth.NewAuthService(db, logger)
+	server.authService = auth.NewAuthService(db, logger, config)
 
 	// Setup initial root account if needed (Issue #3)
 	if config.AuthEnabled {