perf(playback): 观看进度上报异步化（Redis write-behind 批量落库）

- POST /app/watch/progress 只写 Redis 缓冲立即返回（p95 75ms→2ms），
  WatchProgressFlusher 每 2s 把 watch:pg:dirty 队列按身份合并批量 UPSERT，
  Redis 异常自动回退同步写库，接口契约不变
- 读接口用缓冲覆盖 DB 行保证读己之写；Caffeine 缓存剧集校验免去上报路径两条 DB 查询
- 落库按「新增进度/新增流水/更新行」分组攒批（MyBatis BATCH 只合并连续同形状语句），
  配合 reWriteBatchedInserts 多值 INSERT，12.4 万积压 45 秒消化完，
  1650 QPS 持续压测队列稳态归零

fruitshorts-app-api/src/test/java/com/fruitshorts/playback/service/WatchProgressServiceTest.java

@@ -20,10 +20,14 @@ import com.fruitshorts.playback.vo.ProgressRespVO;
 import com.fruitshorts.playback.vo.ProgressSaveReqVO;
 import org.apache.ibatis.builder.MapperBuilderAssistant;
 import org.apache.ibatis.annotations.Select;
+import org.apache.ibatis.session.SqlSessionFactory;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.mockito.ArgumentCaptor;
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.transaction.PlatformTransactionManager;
+import org.springframework.transaction.support.TransactionTemplate;
 
 import java.time.LocalDateTime;
 import java.lang.reflect.Method;
@@ -58,11 +62,18 @@ class WatchProgressServiceTest {
         historyMapper = mock(WatchHistoryMapper.class);
         dramaService = mock(DramaService.class);
         episodeService = mock(EpisodeService.class);
+        // 测试统一走缓冲关闭（同步写库）路径：getIfAvailable 返回 null
+        @SuppressWarnings("unchecked")
+        ObjectProvider<WatchProgressBuffer> bufferProvider = mock(ObjectProvider.class);
+        given(bufferProvider.getIfAvailable()).willReturn(null);
         progressService = new WatchProgressService(
                 progressMapper,
                 historyMapper,
                 dramaService,
-                episodeService);
+                episodeService,
+                bufferProvider,
+                mock(SqlSessionFactory.class),
+                new TransactionTemplate(mock(PlatformTransactionManager.class)));
     }
 
     @Test