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/processors/analyze.py

@@ -226,6 +226,28 @@ class AnalyzeProcessor(BaseProcessor):
             base_url=base_url
         )
 
+    def _normalize_metadata(self, metadata: Dict) -> Dict:
+        """标准化元数据结构（扁平化嵌套结构）
+
+        Args:
+            metadata: 原始元数据字典，可能是嵌套结构
+
+        Returns:
+            扁平化的元数据字典
+        """
+        if not metadata:
+            return {}
+
+        # 如果已经是扁平结构（顶层有 title）
+        if 'title' in metadata:
+            return metadata
+
+        # 处理嵌套结构 {metadata: {...}, stats: {...}}
+        if 'metadata' in metadata:
+            return metadata['metadata'].copy()
+
+        return metadata
+
     def _write_analysis_report(
         self,
         output_path: Path,
@@ -243,8 +265,11 @@ class AnalyzeProcessor(BaseProcessor):
         """
         from kb_deep_analyzer.deep_analyzer import generate_frontmatter
 
+        # 标准化元数据（扁平化嵌套结构）
+        normalized_metadata = self._normalize_metadata(metadata)
+
         # 生成 frontmatter
-        frontmatter = generate_frontmatter(metadata, result, content_type)
+        frontmatter = generate_frontmatter(normalized_metadata, result, content_type)
 
         # 获取完整分析内容
         full_analysis = result.full_analysis