feat: 打手赠送会员（免费接单）独立表与抢单资格

新增 dashou_zengsong_huiyuan，与购买会员分离；抢单/抢单池认赠送有效会员；后台送/改期/移除接口，送时积分非10则改为10并写操作日志。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/view.py

@@ -36,6 +36,7 @@ from .views.kefu import (
     KefuGetDashouListView, KefuGetDashouDetailView, KefuUpdateDashouView,
     KefuGetShangjiaListView, KefuGetShangjiaDetailView, KefuUpdateShangjiaView,
     KefuChatPermissionsView,
+    DashouGiftHuiyuanSaveView, DashouGiftHuiyuanRemoveView,
 )
 from .views.products import (
     GetProductBaseDataView, GetProductListView, SaveProductOrderView,
@@ -96,6 +97,7 @@ __all__ = [
     'GetAdminRolesView', 'GetAdminUserListView', 'ModifyAdminUserView', 'AddAdminUserView',
     # kefu
     'KefuGetDashouListView', 'KefuGetDashouDetailView', 'KefuUpdateDashouView',
+    'DashouGiftHuiyuanSaveView', 'DashouGiftHuiyuanRemoveView',
     'KefuGetShangjiaListView', 'KefuGetShangjiaDetailView', 'KefuUpdateShangjiaView',
     'KefuChatPermissionsView',
     # products