fix(download): return author name instead of mid in _extract_author

- Fix download processor to return actual author name (e.g. "HOLA小测佬")
  instead of B站用户ID (mid, e.g. "406636263")
- Add 5 test cases for author extraction logic
- Resolves report display issue where UP主名称 showed numeric ID

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

kb_workflow/pipeline/processors/download.py

@@ -216,13 +216,13 @@ class DownloadProcessor(BaseProcessor):
             platform: Platform name
 
         Returns:
-            Author string
+            Author string (author name, not mid)
         """
         if platform == "bilibili":
-            # Bilibili metadata structure: {metadata: {mid: ...}, ...}
+            # Bilibili metadata structure: {metadata: {author: ..., mid: ...}, ...}
             if "metadata" in metadata:
-                return str(metadata["metadata"].get("mid", ""))
-            return str(metadata.get("mid", ""))
+                return str(metadata["metadata"].get("author", ""))
+            return str(metadata.get("author", ""))
         else:
             return metadata.get("author", "")