Add PDF serving API with multimedia integration

• Add parent_uuid and media_uuid fields to PretraitementDocument model
  • Create MultimediaAPIService for dynamic folder UUID retrieval
  • Implement PDF folder support (media -> pdf -> {user_id} structure)
  • Add serve_pdf_file API endpoint for PDF file access
  • Update pretraitement service to populate UUID fields from multimedia API
  • Add PDF serving URL pattern to urls.py
  • Include parent_uuid and media_uuid in document serialization
  • Add fallback search strategies for PDF document lookup

Closes IA-436