feat(public-todo): 新增免登录公开待办看板后端

- 新增表 t_public_todo 及 Entity/Mapper/Service/Controller 四个类
- 对外接口 7 个：list/add/update/updateStatus/remove/removeDone/stats
- Controller 不继承 BaseController，不依赖登录态
- WebSecurityConfig 放行 /public-todo/**，与个人待办 /todo/** 隔离
- 落库脚本 sql/public_todo.sql（幂等建表 + 示例数据）

src/main/java/com/freedom/admin/service/impl/PublicTodoServiceImpl.java

@@ -0,0 +1,19 @@
+package com.freedom.admin.service.impl;
+
+import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
+import com.freedom.admin.entity.PublicTodo;
+import com.freedom.admin.mapper.PublicTodoMapper;
+import com.freedom.admin.service.PublicTodoService;
+import lombok.extern.log4j.Log4j2;
+import org.springframework.stereotype.Service;
+
+/**
+ * 业务层实现-公开待办看板表
+ *
+ * @author zhangjiaxun
+ */
+@Service
+@Log4j2
+public class PublicTodoServiceImpl extends ServiceImpl<PublicTodoMapper, PublicTodo> implements PublicTodoService {
+
+}