fix(asr): use content_dir for subtitle output instead of video_path

- Add content_dir field to ASRInput schema
- Pass content_dir from pipeline when creating ASRInput
- Modify _try_official_subtitle to use content_dir for output
- Modify _run_whisper_basic to use content_dir for output
- Add validation for empty video_path in Whisper fallback

Root cause: DownloadProcessor sets content_file="" for video types,
causing Path("").parent to resolve to current directory instead of
content directory. Subtitle files were written to wrong location.

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

kb_pipeline/schemas.py

@@ -198,11 +198,13 @@ class ASRInput(StepInput):
     继承 StepInput 的基础字段，添加视频路径和内容类型。
 
     Attributes:
-        video_path: 视频文件路径
+        video_path: 视频文件路径（可能为空，用于 Whisper 降级）
         content_type: 内容类型，默认为 'video'
+        content_dir: 内容目录路径，用于输出字幕文件
     """
-    video_path: str = Field(..., description="视频文件路径")
+    video_path: str = Field(default="", description="视频文件路径（可能为空）")
     content_type: str = Field(default="video", description="内容类型")
+    content_dir: str = Field(..., description="内容目录路径")
 
 
 class ASROutput(StepOutput):