fix(bilibili): correct metadata and filename in archive

- Add _normalize_metadata() to flatten nested metadata structure
- Fix generate_frontmatter() to use correct metadata fields
- Change _sanitize_filename() to return standardized "bilibili_{bvid}" format

Fixes: analysis.md frontmatter showing "未命名"/0/"未知" instead of actual values
Fixes: audio filename using video title instead of platform_bvid format

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

kb_pipeline/platforms/bilibili.py

@@ -461,26 +461,17 @@ class BilibiliAdapter(PlatformAdapter):
         return "video"
 
     def _sanitize_filename(self, title: str, bvid: str) -> str:
-        """Sanitize title for use as filename.
+        """Generate standardized filename for video download.
 
         Args:
-            title: Video title
-            bvid: Video BV号 for fallback
+            title: Video title (unused, kept for API compatibility)
+            bvid: Video BV号
 
         Returns:
-            Safe filename string
+            Standardized filename in format "bilibili_{bvid}"
         """
-        # Keep alphanumeric, spaces, and common Chinese-safe chars
-        safe_title = "".join(
-            c for c in title
-            if c.isalnum() or c in (' ', '-', '_', '丨', '｜')
-        ).strip()[:50]
-
-        # Use bvid if title is empty
-        if not safe_title:
-            safe_title = f"video_{bvid}"
-
-        return safe_title
+        # Use standardized naming convention: platform_bvid
+        return f"bilibili_{bvid}"
 
     def _create_cookie_file(self, cookie: str) -> str:
         """Create Netscape format cookie file for yt-dlp.