feat: 分组管理页面新增专属倍率管理

- 后端新增 GET /admin/groups/:id/rate-multipliers API
- 前端新增 GroupRateMultipliersModal 组件，支持查看/添加/修改/删除用户专属倍率
- 分组列表操作列新增"专属倍率"按钮
- 修复 antigravity_gateway_service_test.go 参数不匹配的预存问题

backend/internal/service/admin_service.go

@@ -42,6 +42,7 @@ type AdminService interface {
 	UpdateGroup(ctx context.Context, id int64, input *UpdateGroupInput) (*Group, error)
 	DeleteGroup(ctx context.Context, id int64) error
 	GetGroupAPIKeys(ctx context.Context, groupID int64, page, pageSize int) ([]APIKey, int64, error)
+	GetGroupRateMultipliers(ctx context.Context, groupID int64) ([]UserGroupRateEntry, error)
 	UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error
 
 	// API Key management (admin)
@@ -1263,6 +1264,13 @@ func (s *adminServiceImpl) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return keys, result.Total, nil
 }
 
+func (s *adminServiceImpl) GetGroupRateMultipliers(ctx context.Context, groupID int64) ([]UserGroupRateEntry, error) {
+	if s.userGroupRateRepo == nil {
+		return nil, nil
+	}
+	return s.userGroupRateRepo.GetByGroupID(ctx, groupID)
+}
+
 func (s *adminServiceImpl) UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
 	return s.groupRepo.UpdateSortOrders(ctx, updates)
 }