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/handler/admin/admin_service_stub_test.go

@@ -175,6 +175,10 @@ func (s *stubAdminService) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return s.apiKeys, int64(len(s.apiKeys)), nil
 }
 
+func (s *stubAdminService) GetGroupRateMultipliers(_ context.Context, _ int64) ([]service.UserGroupRateEntry, error) {
+	return nil, nil
+}
+
 func (s *stubAdminService) ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string, groupID int64) ([]service.Account, int64, error) {
 	return s.accounts, int64(len(s.accounts)), nil
 }