- Added `TransactionReceipt` model to store payment receipts uploaded by users.
- Introduced endpoints for uploading receipts and listing receipts associated with transactions.
- Updated `TransactionParticipant` model to include a new status 'waiting_approval'.
- Enhanced serializers to handle receipt uploads and retrievals, including validation for file uploads.
- Updated admin interface to manage transaction receipts effectively.
- Added comprehensive API documentation for transaction receipt operations.