- Introduced the FileAccessDownloadToken entity with properties for managing file access tokens.
- Created a migration to add the file_access_download_tokens table to the database with appropriate constraints and indexes.
- Updated OpenAPI documentation to include new OAuth2 usage types: `send_api`.
- Added endpoints for issuing one-click unsubscribe tokens in both single and batch requests.
- Modified OAuth client creation and management to enforce new usage types and redirect URI requirements.
- Implemented logic in the Newsletter service to handle one-click unsubscribe token issuance.
- Updated UI to reflect changes in OAuth client usage options and redirect URI handling.
- Enhanced token generation logic to support new scopes and audience settings for Send Engine.
