Based on the following functional requirements for a library management system, generate a complete OpenAPI 3.0 specification in YAML format. Include all paths, methods (GET, POST, PUT, DELETE), parameters, request bodies, and response schemas. Ensure error handling (4xx, 5xx) is documented. Requirements: Users can register, login, and borrow books. Admins can add and remove books. Books have ISBN, title, author, and availability status.