refactor(media): 抽出 MediaStorage 接口为对象存储切换铺路

本地磁盘实现 MediaStorageService 改为实现接口，五处注入点
改为依赖接口；行为不变。后期新增 S3 实现以配置切换即可。

fruitshorts-transcode-worker/src/test/java/com/fruitshorts/worker/MediaServiceTranscodePipelineTest.java

@@ -8,6 +8,7 @@ import com.fruitshorts.media.dal.entity.MediaUploadTask;
 import com.fruitshorts.media.dal.mapper.MediaAssetMapper;
 import com.fruitshorts.media.dal.mapper.MediaUploadTaskMapper;
 import com.fruitshorts.media.service.MediaService;
+import com.fruitshorts.media.service.MediaStorage;
 import com.fruitshorts.media.service.MediaStorageService;
 import com.fruitshorts.media.service.EpisodeVideoSourcePort;
 import com.fruitshorts.media.service.TranscodeService;
@@ -63,7 +64,7 @@ class MediaServiceTranscodePipelineTest {
         givenCompletedUpload("video.mp4");
         given(storageService.countChunks("upload-1")).willReturn(1);
         given(storageService.merge("upload-1", "episode/202610/video.mp4", 1))
-                .willReturn(new MediaStorageService.MergeResult(
+                .willReturn(new MediaStorage.MergeResult(
                         Path.of("/tmp/video.mp4"), 1024L, "md5"));
         given(storageService.toPublicUrl("episode/202610/video.mp4"))
                 .willReturn(SOURCE_URL);
@@ -79,7 +80,7 @@ class MediaServiceTranscodePipelineTest {
         givenCompletedUpload("cover.jpg");
         given(storageService.countChunks("upload-1")).willReturn(1);
         given(storageService.merge("upload-1", "episode/202610/cover.jpg", 1))
-                .willReturn(new MediaStorageService.MergeResult(
+                .willReturn(new MediaStorage.MergeResult(
                         Path.of("/tmp/cover.jpg"), 512L, "md5"));
         given(storageService.toPublicUrl("episode/202610/cover.jpg"))
                 .willReturn("/files/episode/202610/cover.jpg");