feat(admin): 管理端在线监控 + 好友关系管理后端

- admin/service/online_service.go: 在线用户列表/计数
- admin/controller/online_controller.go: GET /admin/online/users|count
- admin/service/contact_manage_service.go: 好友关系分页查询/管理员删除
- admin/controller/contact_manage_controller.go: GET|DELETE /admin/contacts
- 更新 admin router/provider/wire 集成新控制器

Made-with: Cursor

backend/go-service/app/admin/provider.go

@@ -14,4 +14,8 @@ var AdminSet = wire.NewSet(
 	dao.NewUserManageDAO,
 	service.NewUserManageService,
 	controller.NewUserManageController,
+	service.NewOnlineManageService,
+	controller.NewOnlineController,
+	service.NewContactManageService,
+	controller.NewContactManageController,
 )