feat(download): add video download capability for videos without official subtitles

- Add download_video() to BilibiliAdapter using yt-dlp
- Add bilibili_cookie config to Settings
- Update DownloadProcessor to call video download for bilibili videos
- Add idempotency check to ASRProcessor for existing subtitle files
- Fix indentation error in asr.py

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

kb_pipeline/schemas.py

@@ -291,9 +291,11 @@ class DownloadInput(StepInput):
     Attributes:
         content_dir: 保存下载内容的目录路径
         content_type: 内容类型 (video, article, answer, zhuanlan)
+        source_url: 原始 URL（可选，用于知乎回答等需要完整 URL 的情况）
     """
     content_dir: str = Field(..., min_length=1, description="Directory to save downloaded content")
     content_type: str = Field(..., min_length=1, description="Content type (video, article, answer, zhuanlan)")
+    source_url: Optional[str] = Field(None, description="Original source URL")
 
 
 class DownloadOutput(StepOutput):