feat(复习): 节点级 focusPath + findNode，回退 session 维度

- 移除 MindMapNode.sessionNum、filterBySession、getTaskSessions
- 新增 MindMapTreeTool.extractSubtree() 按路径提取子树作为对比基准
- 新增 findClosestNode/getPath/similarityScore 支持节点匹配
- 新增 POST /review/standard-mind-map/{taskNum}/find-node
- recallCompare 使用 focusPath 替代 sessionNum
- ReviewRecallRecordEntity.focusPath 替代 sessionNum
- Flyway V20260706_1: review_recall_records 加 focus_path 列

src/main/java/com/guo/learningprogresstracker/utils/MindMapNode.java

@@ -35,8 +35,6 @@ public class MindMapNode {
     private String notes;
     private String sourceType;
     private Integer sourceId;
-    /** 可选：节点来源会话编码（REPORT/FRAGMENT 节点使用） */
-    private String sessionNum;
     private List<MindMapNode> children;
 
     public MindMapNode(String title) {
@@ -53,7 +51,6 @@ public class MindMapNode {
         map.put("notes", notes != null ? notes : "");
         if (sourceType != null) map.put("sourceType", sourceType);
         if (sourceId != null) map.put("sourceId", sourceId);
-        if (sessionNum != null) map.put("sessionNum", sessionNum);
         List<Object> childMaps = new ArrayList<>();
         if (children != null) {
             for (MindMapNode c : children) {
@@ -73,7 +70,6 @@ public class MindMapNode {
         if (map.containsKey("sourceId") && map.get("sourceId") != null) {
             node.setSourceId(((Number) map.get("sourceId")).intValue());
         }
-        node.setSessionNum((String) map.get("sessionNum"));
         Object raw = map.getOrDefault("children", map.get("nodes"));
         List<MindMapNode> children = new ArrayList<>();
         if (raw instanceof List<?> list) {