- Remove merge_metadata and its helper functions from metadata_utils.py
- Pass existing_metadata to MetadataExtractor.extract_metadata() as LLM context
- Add merge instructions to extract_user_metadata.jinja2 prompt (zh/en)
- Update Celery task to read existing metadata before extraction and overwrite
- Simplify field descriptions in UserMetadataProfile model
- Add _update_timestamps helper to track changed fields