feat(会话)：分页查询任务的历史残片/报告接口

StudySessionsService 新增：
- getTaskFragments(taskNum, page, size, keyword) → Page<FragmentsEntity>
- getTaskReports(taskNum, page, size, keyword) → Page<ReportsEntity>

通过子查询 session_num in (select session_num from study_sessions
where task_num = ?) 实现跨 session 聚合，keyword 支持 content
字段 LIKE 搜索。接口使用 MyBatis-Plus Page 分页。

控制器新增两个 GET 端点：
- /study-sessions/tasks/{taskNum}/fragments
- /study-sessions/tasks/{taskNum}/reports

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

@@ -1,6 +1,7 @@
 package com.guo.learningprogresstracker.service.impl;
 
 import com.baomidou.mybatisplus.core.toolkit.Wrappers;
+import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
 import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
 import com.google.protobuf.ServiceException;
 import com.guo.learningprogresstracker.dto.StudySessionsDto;
@@ -190,4 +191,30 @@ public class StudySessionsServiceImpl extends ServiceImpl<StudySessionsMapper, S
                 .orElseGet(() -> String.join("\n", fragments));
     }
 
+    // ============ 分页查询任务的历史残片/报告 ============
+
+    @Override
+    public Page<StudyReportFragmentsEntity> getTaskFragments(String taskNum, int page, int size, String keyword) {
+        Page<StudyReportFragmentsEntity> pg = new Page<>(page, size);
+        var wrapper = Wrappers.lambdaQuery(StudyReportFragmentsEntity.class)
+                .apply("session_num in (select session_num from study_sessions where task_num = {0})", taskNum)
+                .orderByDesc(StudyReportFragmentsEntity::getCreatedTime);
+        if (keyword != null && !keyword.isBlank()) {
+            wrapper.like(StudyReportFragmentsEntity::getContent, keyword);
+        }
+        return studyReportFragmentsMapper.selectPage(pg, wrapper);
+    }
+
+    @Override
+    public Page<StudyReportsEntity> getTaskReports(String taskNum, int page, int size, String keyword) {
+        Page<StudyReportsEntity> pg = new Page<>(page, size);
+        var wrapper = Wrappers.lambdaQuery(StudyReportsEntity.class)
+                .apply("session_num in (select session_num from study_sessions where task_num = {0})", taskNum)
+                .orderByDesc(StudyReportsEntity::getCreatedTime);
+        if (keyword != null && !keyword.isBlank()) {
+            wrapper.like(StudyReportsEntity::getContent, keyword);
+        }
+        return studyReportsMapper.selectPage(pg, wrapper);
+    }
+
 }