Fix links.json to support multiple source articles per URL and resolve url_mapping.json empty file issue
- Changed links.json structure to map URLs to lists of atom IDs instead of single IDs
- Added proper deduplication to avoid storing duplicate entry IDs for same URL
- Fixed url_mapping.json by converting HttpUrl objects to strings for proper comparison
- Added debug output to show registered URLs count for troubleshooting
- Updated filtering logic to handle new list-based link structure
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Co-Authored-By: Claude <noreply@anthropic.com>