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
 }

backend/internal/handler/admin/group_handler.go

@@ -339,6 +339,27 @@ func (h *GroupHandler) GetGroupAPIKeys(c *gin.Context) {
 	response.Paginated(c, outKeys, total, page, pageSize)
 }
 
+// GetGroupRateMultipliers handles getting rate multipliers for users in a group
+// GET /api/v1/admin/groups/:id/rate-multipliers
+func (h *GroupHandler) GetGroupRateMultipliers(c *gin.Context) {
+	groupID, err := strconv.ParseInt(c.Param("id"), 10, 64)
+	if err != nil {
+		response.BadRequest(c, "Invalid group ID")
+		return
+	}
+
+	entries, err := h.adminService.GetGroupRateMultipliers(c.Request.Context(), groupID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	if entries == nil {
+		entries = []service.UserGroupRateEntry{}
+	}
+	response.Success(c, entries)
+}
+
 // UpdateSortOrderRequest represents the request to update group sort orders
 type UpdateSortOrderRequest struct {
 	Updates []struct {