- Add user_id field to PretraitementDocument model for user-based file organization

• Implement upload_pdf_file method with file validation, size limits (50MB), and unique filename generation
  • Create serve_media_file API endpoint to serve PDF documents with URL decoding support
  • Update DocumentUploadController to accept user_id as required parameter
  • Add file organization structure: media/documents/{user_id}/{timestamp}{uuid}{filename}
  • Clean filename generation: remove spaces and special characters from uploaded files
  • Add URL route /api/core-ia/media/{user_id}/{file_name} for document access
  • Support multiple file formats (PDF, DOCX, DOC, TXT, images) in media serving
  • Include security checks for path traversal prevention
  • Add database migration for user_id field with default value 1 for existing records

Closes IA-436