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/platforms/zhihu.py

@@ -10,6 +10,7 @@ Provides content extraction for Zhihu platform:
 from typing import Dict, Any, Optional
 from datetime import datetime
 import re
+import asyncio
 
 from kb_pipeline.platforms.base import PlatformAdapter
 from kb_pipeline.errors import AuthError
@@ -146,12 +147,13 @@ class ZhihuAdapter(PlatformAdapter):
 
         return False
 
-    async def fetch_metadata(self, content_id: str, content_type: str) -> Dict[str, Any]:
+    async def fetch_metadata(self, content_id: str, content_type: str, source_url: str = None) -> Dict[str, Any]:
         """Fetch metadata for the content.
 
         Args:
             content_id: Answer ID or article ID
             content_type: 'answer' or 'zhuanlan'
+            source_url: Original source URL (optional, used for answers to get question ID)
 
         Returns:
             Dictionary containing metadata (title, author, publish_time, etc.)
@@ -169,15 +171,19 @@ class ZhihuAdapter(PlatformAdapter):
 
         # Build URL based on content type
         if content_type == "answer":
-            # For answers, we need the question ID too, but extractor handles this
-            url = f"https://www.zhihu.com/question/0/answer/{content_id}"
+            # For answers, try to use source_url first, then fallback to constructed URL
+            if source_url:
+                url = source_url
+            else:
+                # Fallback: use answer ID only (may not work for all cases)
+                url = f"https://www.zhihu.com/question/0/answer/{content_id}"
         elif content_type == "zhuanlan":
             url = f"https://zhuanlan.zhihu.com/p/{content_id}"
         else:
             raise ValueError(f"Unsupported content type: {content_type}")
 
-        # Extract content
-        result = self._extract_with_playwright(url)
+        # Extract content in a separate thread to avoid blocking asyncio
+        result = await asyncio.to_thread(self._extract_with_playwright, url)
 
         # Check for login page
         if self._detect_login_page(result):
@@ -198,12 +204,13 @@ class ZhihuAdapter(PlatformAdapter):
 
         return result
 
-    async def fetch_content(self, content_id: str, content_type: str) -> str:
+    async def fetch_content(self, content_id: str, content_type: str, source_url: str = None) -> str:
         """Fetch content in Markdown format.
 
         Args:
             content_id: Answer ID or article ID
             content_type: 'answer' or 'zhuanlan'
+            source_url: Original source URL (optional, used for answers)
 
         Returns:
             Markdown-formatted content with YAML frontmatter
@@ -212,7 +219,7 @@ class ZhihuAdapter(PlatformAdapter):
             AuthError: If cookie is missing or expired
         """
         # Get metadata (which also validates auth)
-        metadata = await self.fetch_metadata(content_id, content_type)
+        metadata = await self.fetch_metadata(content_id, content_type, source_url=source_url)
 
         # Get content markdown
         content_markdown = metadata.get("content_markdown", "") or metadata.get("content", "")