Includes detailed OpenAPI schemas for unsubscribe (GET and POST) and tracking pixel (GET) endpoints, supporting email compatibility and event tracking. Added support for RFC 8058-compliant one-click unsubscribe functionality and transparent image-based email tracking.
Fixes: 1) Fix missing import for `send_message` moved to method scope in Telegram message handler; 2) Correct Swagger UI socket connection setting to `False`.
Extra: 1) Minor code cleanup and reformatting in viewsets and settings.
Fixes: 1) Replace generic serializers with context-specific implementations in schema definitions.
Extra: 1) Minor code cleanup and reorganization for improved readability and maintainability; 2) Add comments for new serializers.
