feat(预期)：学习预期后端闭环——实体/服务/端点

- 迁移：study_expectations.session_num 改为 varchar 对齐会话编码
- StudyExpectationsEntity / Mapper / Service：每会话一条预期，重复创建覆盖
- StudySessionController：PUT/GET /{sessionNum}/expectation
- 移除空的 studySessionList 占位方法

src/main/java/com/guo/learningprogresstracker/mapper/StudyExpectationsMapper.java

@@ -0,0 +1,9 @@
+package com.guo.learningprogresstracker.mapper;
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper;
+import com.guo.learningprogresstracker.entity.StudyExpectationsEntity;
+import org.apache.ibatis.annotations.Mapper;
+
+@Mapper
+public interface StudyExpectationsMapper extends BaseMapper<StudyExpectationsEntity> {
+}