feat: add RAG converter utilities and update integration guide
RAG Converter:
- Create app/utils/rag_converter.py with conversion functions
- parsed_doc_to_haystack_docs() - convert ParsedDocument to Haystack format
- parsed_chunks_to_haystack_docs() - convert ParsedChunk list to Haystack
- validate_parsed_doc_for_rag() - validate required fields before conversion
- Automatic metadata extraction (dao_id, doc_id, page, block_type)
- Preserve optional fields (bbox, section, reading_order)
Integration Guide:
- Update with ready-to-use converter functions
- Add validation examples
- Complete workflow examples
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