feat(复习): 支持会话维度复习 + 标准导图按会话过滤（后端）

- MindMapNode 新增 sessionNum 字段
- BuiltinMindMapGenerator 生成节点时填充 sessionNum
- MindMapTreeTool.filterBySession() 按会话过滤标准导图
- recallCompare 接受 sessionNum 参数
- ReviewRecallRecordEntity 新增 sessionNum 字段
- Flyway V20260705_1: review_recall_records 加 session_num 列
- 新增 GET /study-sessions/tasks/{taskNum}/sessions 端点

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

@@ -96,7 +96,7 @@ public class StandardMindMapServiceImpl implements StandardMindMapService {
 
     @Override
     @Transactional
-    public ReviewStandardMindMapEntity recallCompare(String taskNum, String recallOutline)
+    public ReviewStandardMindMapEntity recallCompare(String taskNum, String recallOutline, String sessionNum)
             throws NotFindEntitiesException, OperationFailedException {
         ensureTaskExists(taskNum);
 
@@ -107,6 +107,11 @@ public class StandardMindMapServiceImpl implements StandardMindMapService {
         MindMapNode standardRoot = MindMapTreeTool.fromJson(standard.getContent(), objectMapper);
         MindMapNode recallRoot = MindMapTreeTool.parseOutline(recallOutline);
 
+        // 2b. 若指定了会话维度，过滤标准导图只保留该会话相关节点
+        if (sessionNum != null && !sessionNum.isBlank()) {
+            standardRoot = MindMapTreeTool.filterBySession(standardRoot, sessionNum);
+        }
+
         // 3. 执行对比（优先 AI 语义对比，失败时降级为内置算法）
         CompareResult result;
         if (aiServiceClient.isConfigured()) {
@@ -142,6 +147,7 @@ public class StandardMindMapServiceImpl implements StandardMindMapService {
         ReviewRecallRecordEntity record = new ReviewRecallRecordEntity();
         record.setTaskNum(taskNum);
         record.setStandardMapId(standard.getId());
+        record.setSessionNum(sessionNum != null && !sessionNum.isBlank() ? sessionNum : null);
         record.setRecallContent(recallOutline);
         record.setCompareResult(resultJson);
         record.setRecallRatio(result.getRecallRatio());