feat(domain,presentation,tests): implement Relay entity, DTOs, and API errors

- Add Relay entity with constructors and business logic methods
- Add RelayDto for API responses with From<Relay> conversion
- Add ApiError with ResponseError trait for unified error handling
- Add dependency injection tests for mock infrastructure in test mode

backend/src/presentation/dto/mod.rs

@@ -0,0 +1,6 @@
+/// Relay-specific Data Transfer Objects.
+///
+/// This module contains DTO structures for relay-related API responses,
+/// providing serialized representations of relay domain objects for
+/// external consumption.
+pub mod relay_dto;