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/controller/StudySessionController.java

@@ -2,9 +2,12 @@ package com.guo.learningprogresstracker.controller;
 
 import com.google.protobuf.ServiceException;
 import com.guo.learningprogresstracker.dto.request.EndedStudySessionRequest;
+import com.guo.learningprogresstracker.dto.request.UpsertExpectationRequest;
 import com.guo.learningprogresstracker.dto.response.StudySessionResponse;
 import com.guo.learningprogresstracker.entity.CommonResult;
+import com.guo.learningprogresstracker.entity.StudyExpectationsEntity;
 import com.guo.learningprogresstracker.exception.ErrorParameterException;
+import com.guo.learningprogresstracker.service.StudyExpectationsService;
 import com.guo.learningprogresstracker.service.impl.StudySessionsServiceImpl;
 import jakarta.validation.Valid;
 import jakarta.validation.constraints.NotEmpty;
@@ -26,9 +29,25 @@ public class StudySessionController {
 
     private final StudySessionsServiceImpl studySessionsServiceImpl;
 
-    @PostMapping
-    public void studySessionList() {
+    private final StudyExpectationsService studyExpectationsService;
 
+    /**
+     * 创建/更新学习会话的学习预期
+     */
+    @PutMapping("/{sessionNum}/expectation")
+    public CommonResult<StudyExpectationsEntity> upsertExpectation(
+            @PathVariable("sessionNum") String sessionNum,
+            @Valid @RequestBody UpsertExpectationRequest request) throws ErrorParameterException {
+        return CommonResult.success(studyExpectationsService.upsertExpectation(sessionNum, request.getDescription()));
+    }
+
+    /**
+     * 获取学习会话的学习预期
+     */
+    @GetMapping("/{sessionNum}/expectation")
+    public CommonResult<StudyExpectationsEntity> getExpectation(
+            @PathVariable("sessionNum") String sessionNum) {
+        return CommonResult.success(studyExpectationsService.getBySessionNum(sessionNum));
     }
 
     /**

src/main/java/com/guo/learningprogresstracker/dto/request/UpsertExpectationRequest.java

@@ -0,0 +1,14 @@
+package com.guo.learningprogresstracker.dto.request;
+
+import jakarta.validation.constraints.NotBlank;
+import lombok.Data;
+
+/**
+ * 创建/更新学习预期请求
+ */
+@Data
+public class UpsertExpectationRequest {
+
+    @NotBlank(message = "学习预期不可为空")
+    private String description;
+}

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;
+}

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> {
+}

src/main/java/com/guo/learningprogresstracker/service/StudyExpectationsService.java

@@ -0,0 +1,20 @@
+package com.guo.learningprogresstracker.service;
+
+import com.guo.learningprogresstracker.entity.StudyExpectationsEntity;
+import com.guo.learningprogresstracker.exception.ErrorParameterException;
+
+/**
+ * 学习预期服务
+ */
+public interface StudyExpectationsService {
+
+    /**
+     * 为学习会话创建学习预期（每个会话最多一条，重复创建则覆盖）
+     */
+    StudyExpectationsEntity upsertExpectation(String sessionNum, String description) throws ErrorParameterException;
+
+    /**
+     * 查询学习会话的学习预期，不存在时返回 null
+     */
+    StudyExpectationsEntity getBySessionNum(String sessionNum);
+}

src/main/java/com/guo/learningprogresstracker/service/impl/StudyExpectationsServiceImpl.java

@@ -0,0 +1,52 @@
+package com.guo.learningprogresstracker.service.impl;
+
+import com.baomidou.mybatisplus.core.toolkit.Wrappers;
+import com.guo.learningprogresstracker.entity.StudyExpectationsEntity;
+import com.guo.learningprogresstracker.entity.StudySessionsEntity;
+import com.guo.learningprogresstracker.exception.ErrorParameterException;
+import com.guo.learningprogresstracker.mapper.StudyExpectationsMapper;
+import com.guo.learningprogresstracker.mapper.StudySessionsMapper;
+import com.guo.learningprogresstracker.service.StudyExpectationsService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+/**
+ * 学习预期服务实现
+ */
+@Service
+@RequiredArgsConstructor
+public class StudyExpectationsServiceImpl implements StudyExpectationsService {
+
+    private final StudyExpectationsMapper studyExpectationsMapper;
+    private final StudySessionsMapper studySessionsMapper;
+
+    @Override
+    public StudyExpectationsEntity upsertExpectation(String sessionNum, String description) throws ErrorParameterException {
+        boolean sessionExists = studySessionsMapper.exists(
+                Wrappers.<StudySessionsEntity>lambdaQuery()
+                        .eq(StudySessionsEntity::getSessionNum, sessionNum));
+        if (!sessionExists) {
+            throw new ErrorParameterException("学习会话[" + sessionNum + "]不存在");
+        }
+
+        StudyExpectationsEntity existing = getBySessionNum(sessionNum);
+        if (existing == null) {
+            StudyExpectationsEntity entity = new StudyExpectationsEntity();
+            entity.setSessionNum(sessionNum);
+            entity.setDescription(description);
+            studyExpectationsMapper.insert(entity);
+            return entity;
+        }
+        existing.setDescription(description);
+        studyExpectationsMapper.updateById(existing);
+        return existing;
+    }
+
+    @Override
+    public StudyExpectationsEntity getBySessionNum(String sessionNum) {
+        return studyExpectationsMapper.selectOne(
+                Wrappers.<StudyExpectationsEntity>lambdaQuery()
+                        .eq(StudyExpectationsEntity::getSessionNum, sessionNum)
+                        .last("LIMIT 1"));
+    }
+}

src/main/resources/db/migration/V20260703_1__alter_study_expectations_session_num.sql

@@ -0,0 +1,2 @@
+alter table study_expectations
+    modify session_num varchar(255) not null comment '会话编码';