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));
     }
 
     /**