feat(复习)：feed 智能模式——加权随机采样

- /review/feed?mode=smart：时间衰减 × 回忆掌握度加权采样
- 越久未见、回忆覆盖率越低的内容出现概率越高
- 保持随机性，符合复习模块偶遇式设计理念
- 概率性单元测试验证低掌握内容显著优先

src/test/java/com/guo/learningprogresstracker/service/impl/ReviewServiceImplTest.java

@@ -5,6 +5,7 @@ import com.baomidou.mybatisplus.core.metadata.TableInfoHelper;
 import com.guo.learningprogresstracker.dto.ReviewFeedItem;
 import com.guo.learningprogresstracker.dto.request.UpsertReviewMindMapRequest;
 import com.guo.learningprogresstracker.entity.ReviewMindMapEntity;
+import com.guo.learningprogresstracker.entity.ReviewRecallRecordEntity;
 import com.guo.learningprogresstracker.entity.StudyReportFragmentsEntity;
 import com.guo.learningprogresstracker.entity.StudyReportsEntity;
 import com.guo.learningprogresstracker.entity.StudySessionsEntity;
@@ -12,6 +13,7 @@ import com.guo.learningprogresstracker.entity.TaskEntity;
 import com.guo.learningprogresstracker.enums.ReviewMindMapFileFormatEnum;
 import com.guo.learningprogresstracker.enums.ReviewMindMapParseStatusEnum;
 import com.guo.learningprogresstracker.mapper.ReviewMindMapMapper;
+import com.guo.learningprogresstracker.mapper.ReviewRecallRecordMapper;
 import com.guo.learningprogresstracker.mapper.StudyReportFragmentsMapper;
 import com.guo.learningprogresstracker.mapper.StudyReportsMapper;
 import com.guo.learningprogresstracker.mapper.StudySessionsMapper;
@@ -67,6 +69,66 @@ class ReviewServiceImplTest {
     @Mock
     private ReviewMindMapMapper reviewMindMapMapper;
 
+    @Mock
+    private ReviewRecallRecordMapper reviewRecallRecordMapper;
+
+    @Test
+    void getReviewFeed_smartMode_prefersLowRecallRatioContent() {
+        LocalDateTime now = LocalDateTime.now();
+
+        StudyReportsEntity oldLowMastery = new StudyReportsEntity();
+        oldLowMastery.setId(1);
+        oldLowMastery.setSessionNum("S_LOW");
+        oldLowMastery.setContent("很久没复习且掌握度低");
+        oldLowMastery.setCreatedTime(now.minusDays(60));
+
+        StudyReportsEntity freshHighMastery = new StudyReportsEntity();
+        freshHighMastery.setId(2);
+        freshHighMastery.setSessionNum("S_HIGH");
+        freshHighMastery.setContent("刚学完且掌握度高");
+        freshHighMastery.setCreatedTime(now);
+
+        StudySessionsEntity sessionLow = new StudySessionsEntity();
+        sessionLow.setSessionNum("S_LOW");
+        sessionLow.setTaskNum("T_LOW");
+        StudySessionsEntity sessionHigh = new StudySessionsEntity();
+        sessionHigh.setSessionNum("S_HIGH");
+        sessionHigh.setTaskNum("T_HIGH");
+
+        TaskEntity taskLow = new TaskEntity();
+        taskLow.setTaskNum("T_LOW");
+        taskLow.setTaskName("低掌握任务");
+        TaskEntity taskHigh = new TaskEntity();
+        taskHigh.setTaskNum("T_HIGH");
+        taskHigh.setTaskName("高掌握任务");
+
+        ReviewRecallRecordEntity highRatioRecord = new ReviewRecallRecordEntity();
+        highRatioRecord.setTaskNum("T_HIGH");
+        highRatioRecord.setRecallRatio(1.0);
+        highRatioRecord.setCreatedTime(now);
+
+        when(studyReportsMapper.selectList(any())).thenReturn(List.of(oldLowMastery, freshHighMastery));
+        when(studyReportFragmentsMapper.selectList(any())).thenReturn(List.of());
+        when(studySessionsMapper.selectList(any())).thenReturn(List.of(sessionLow, sessionHigh));
+        when(tasksMapper.selectList(any())).thenReturn(List.of(taskLow, taskHigh));
+        when(reviewRecallRecordMapper.selectList(any())).thenReturn(List.of(highRatioRecord));
+
+        List<ReviewFeedItem> items = reviewService.getReviewFeed(1, "smart");
+
+        // 采样是概率性的，多次运行统计低掌握内容被选中的频率应显著更高
+        int lowMasteryWins = 0;
+        for (int i = 0; i < 200; i++) {
+            List<ReviewFeedItem> sample = reviewService.getReviewFeed(1, "smart");
+            if (!sample.isEmpty() && "T_LOW".equals(sample.get(0).getTaskNum())) {
+                lowMasteryWins++;
+            }
+        }
+        assertEquals(1, items.size());
+        // 低掌握内容权重约为高掌握内容的 5 倍以上，200 次中至少应赢 120 次
+        org.junit.jupiter.api.Assertions.assertTrue(lowMasteryWins > 120,
+                "低掌握内容被选中次数应显著更多，实际: " + lowMasteryWins + "/200");
+    }
+
     @Test
     void getReviewFeed_shouldApplyFinalLimitAfterMerge() {
         StudyReportsEntity report = new StudyReportsEntity();