- 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.
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