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/entity/StudyExpectationsEntity.java

@@ -0,0 +1,32 @@
+package com.guo.learningprogresstracker.entity;
+
+import com.baomidou.mybatisplus.annotation.IdType;
+import com.baomidou.mybatisplus.annotation.TableField;
+import com.baomidou.mybatisplus.annotation.TableId;
+import com.baomidou.mybatisplus.annotation.TableName;
+import lombok.Data;
+
+import java.io.Serializable;
+
+/**
+ * 存储每次学习开始前的预期
+ */
+@TableName(value = "study_expectations")
+@Data
+public class StudyExpectationsEntity extends BaseEntity implements Serializable {
+
+    @TableId(value = "expectation_id", type = IdType.AUTO)
+    private Integer expectationId;
+
+    @TableField(value = "session_num")
+    private String sessionNum;
+
+    /**
+     * 学习预期的详细描述
+     */
+    @TableField(value = "description")
+    private String description;
+
+    @TableField(exist = false)
+    private static final long serialVersionUID = 1L;
+}