feat(youtube): 使用 youtube-transcript-api 优先获取字幕，有字幕时跳过音频下载

- 新增 YouTubeSubtitleFetcher 模块，通过 youtube-transcript-api 获取字幕
- 重构笔记生成流程：缓存 → 平台字幕 → 按需下载 → 转写 fallback
- 有字幕时仅提取视频元信息，不下载音视频文件
- 添加 youtube-transcript-api 依赖

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/note.py

@@ -133,8 +133,46 @@ class NoteGenerator:
             audio_cache_file = NOTE_OUTPUT_DIR / f"{task_id}_audio.json"
             transcript_cache_file = NOTE_OUTPUT_DIR / f"{task_id}_transcript.json"
             markdown_cache_file = NOTE_OUTPUT_DIR / f"{task_id}_markdown.md"
-            print(audio_cache_file)
-            # 1. 下载音频/视频
+            # 1. 获取字幕/转写：优先缓存 → 平台字幕 → 音频转写
+            transcript = None
+
+            # 尝试读取缓存
+            if transcript_cache_file.exists():
+                logger.info(f"检测到转写缓存 ({transcript_cache_file})，尝试读取")
+                try:
+                    data = json.loads(transcript_cache_file.read_text(encoding="utf-8"))
+                    segments = [TranscriptSegment(**seg) for seg in data.get("segments", [])]
+                    transcript = TranscriptResult(
+                        language=data.get("language"),
+                        full_text=data["full_text"],
+                        segments=segments,
+                    )
+                    logger.info(f"已从缓存加载转写结果，共 {len(segments)} 段")
+                except Exception as e:
+                    logger.warning(f"加载转写缓存失败: {e}")
+
+            # 缓存没有，尝试获取平台字幕
+            if transcript is None:
+                logger.info("尝试获取平台字幕（优先于音频下载）...")
+                try:
+                    transcript = downloader.download_subtitles(video_url)
+                    if transcript and transcript.segments:
+                        logger.info(f"成功获取平台字幕，共 {len(transcript.segments)} 段")
+                        transcript_cache_file.write_text(
+                            json.dumps(asdict(transcript), ensure_ascii=False, indent=2),
+                            encoding="utf-8",
+                        )
+                    else:
+                        transcript = None
+                        logger.info("平台无可用字幕，将下载音频后转写")
+                except Exception as e:
+                    logger.warning(f"获取平台字幕失败: {e}，将下载音频后转写")
+                    transcript = None
+
+            # 2. 下载音频/视频
+            # 有字幕时只提取元信息，不下载音视频文件（除非需要截图/视频理解）
+            has_transcript = transcript is not None
+            need_full_download = not has_transcript or screenshot or video_understanding
             audio_meta = self._download_media(
                 downloader=downloader,
                 video_url=video_url,
@@ -147,18 +185,19 @@ class NoteGenerator:
                 video_understanding=video_understanding,
                 video_interval=video_interval,
                 grid_size=grid_size,
+                skip_download=not need_full_download,
             )
 
-            # 2. 获取字幕/转写文字
-            # 优先尝试获取平台字幕，没有再 fallback 到音频转写
-            transcript = self._get_transcript(
-                downloader=downloader,
-                video_url=video_url,
-                audio_file=audio_meta.file_path,
-                transcript_cache_file=transcript_cache_file,
-                status_phase=TaskStatus.TRANSCRIBING,
-                task_id=task_id,
-            )
+            # 3. 如果前面没拿到字幕，走转写流程
+            if transcript is None:
+                transcript = self._get_transcript(
+                    downloader=downloader,
+                    video_url=video_url,
+                    audio_file=audio_meta.file_path,
+                    transcript_cache_file=transcript_cache_file,
+                    status_phase=TaskStatus.TRANSCRIBING,
+                    task_id=task_id,
+                )
 
             # 3. GPT 总结
             markdown = self._summarize_text(
@@ -331,6 +370,7 @@ class NoteGenerator:
         video_understanding: bool,
         video_interval: int,
         grid_size: List[int],
+        skip_download: bool = False,
     ) -> AudioDownloadResult | None:
         """
         1. 检查音频缓存；若不存在，则根据需要下载音频或视频（若需截图/可视化）。
@@ -353,7 +393,34 @@ class NoteGenerator:
         task_id = audio_cache_file.stem.split("_")[0]
         self._update_status(task_id, status_phase)
 
+        # 已有缓存，尝试加载
+        if audio_cache_file.exists():
+            logger.info(f"检测到音频缓存 ({audio_cache_file})，直接读取")
+            try:
+                data = json.loads(audio_cache_file.read_text(encoding="utf-8"))
+                return AudioDownloadResult(**data)
+            except Exception as e:
+                logger.warning(f"读取音频缓存失败，将重新下载：{e}")
 
+        # 有字幕且不需要截图/视频理解时，只提取元信息不下载文件
+        if skip_download:
+            logger.info("已有字幕，仅提取视频元信息（不下载音视频）")
+            try:
+                audio = downloader.download(
+                    video_url=video_url,
+                    quality=quality,
+                    output_dir=output_path,
+                    need_video=False,
+                    skip_download=True,
+                )
+                audio_cache_file.write_text(
+                    json.dumps(asdict(audio), ensure_ascii=False, indent=2),
+                    encoding="utf-8",
+                )
+                logger.info(f"元信息提取完成 ({audio_cache_file})")
+                return audio
+            except Exception as exc:
+                logger.warning(f"元信息提取失败，将尝试完整下载: {exc}")
 
         # 判断是否需要下载视频
         need_video = screenshot or video_understanding
@@ -368,9 +435,8 @@ class NoteGenerator:
                 self.video_path = Path(video_path_str)
                 logger.info(f"视频下载完成：{self.video_path}")
 
-                # 若指定了 grid_size，则生成缩略图
                 if grid_size:
-                    self.video_img_urls=VideoReader(
+                    self.video_img_urls = VideoReader(
                         video_path=str(self.video_path),
                         grid_size=tuple(grid_size),
                         frame_interval=frame_interval,
@@ -382,17 +448,9 @@ class NoteGenerator:
                     logger.info("未指定 grid_size，跳过缩略图生成")
             except Exception as exc:
                 logger.error(f"视频下载失败：{exc}")
-
                 self._handle_exception(task_id, exc)
                 raise
-        # 已有缓存，尝试加载
-        if audio_cache_file.exists():
-            logger.info(f"检测到音频缓存 ({audio_cache_file})，直接读取")
-            try:
-                data = json.loads(audio_cache_file.read_text(encoding="utf-8"))
-                return AudioDownloadResult(**data)
-            except Exception as e:
-                logger.warning(f"读取音频缓存失败，将重新下载：{e}")
+
         # 下载音频
         try:
             logger.info("开始下载音频")
@@ -402,7 +460,6 @@ class NoteGenerator:
                 output_dir=output_path,
                 need_video=need_video,
             )
-            # 缓存 audio 元信息到本地 JSON
             audio_cache_file.write_text(json.dumps(asdict(audio), ensure_ascii=False, indent=2), encoding="utf-8")
             logger.info(f"音频下载并缓存成功 ({audio_cache_file})")
             return audio