fix: 修复 NewSettingHandler 参数不足导致的编译错误

- 测试文件添加第三个参数 userService（nil）
- Handler 添加 userService 空指针检查，防止测试环境 panic

backend/internal/server/api_contract_test.go

@@ -383,7 +383,7 @@ func newContractDeps(t *testing.T) *contractDeps {
 	authHandler := handler.NewAuthHandler(nil, userService)
 	apiKeyHandler := handler.NewAPIKeyHandler(apiKeyService)
 	usageHandler := handler.NewUsageHandler(usageService, apiKeyService)
-	adminSettingHandler := adminhandler.NewSettingHandler(settingService, nil)
+	adminSettingHandler := adminhandler.NewSettingHandler(settingService, nil, nil)
 
 	jwtAuth := func(c *gin.Context) {
 		c.Set(string(middleware.ContextKeyUser), middleware.AuthSubject{