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/config.py

@@ -50,6 +50,9 @@ class KBSettings(BaseSettings):
     zhihu_cookie: SecretStr = Field(
         default=SecretStr(""), description="知乎 Cookie"
     )
+    bilibili_cookie: SecretStr = Field(
+        default=SecretStr(""), description="B站 Cookie (for video download)"
+    )
 
 
 @lru_cache