migrate file storage from local to AWS S3 via multimedia API

• Add s3_file_uuid field to PretraitementDocument model for S3 file tracking
  • Create MultimediaService for S3 operations via multimedia API (upload, download, folder management)
  • Add constants.py for centralized API configuration (base URL, folder names)
  • Implement dynamic folder discovery (media/pdf folders) instead of hardcoded UUIDs
  • Update pretraitement_document_service.py to upload files to S3 before processing
  • Add S3-based text extraction and preprocessing methods (_extraire_texte_from_data, pretraiter_texte_from_s3)
  • Remove local file serving endpoint and dependencies from views and URLs
  • Update API responses to include S3 download URLs and file UUIDs
  • Add comprehensive logging for S3 operations and user activity tracking
  • Ensure automatic user folder creation (media/pdf/{user_id}/) if not exists
  • Maintain backward compatibility with existing preprocessing workflows

Closes IA-436