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/mod.rs

@@ -94,3 +94,12 @@
 //! - Architecture: `specs/constitution.md` - API-First Design principle
 //! - API design: `specs/001-modbus-relay-control/plan.md` - Presentation layer tasks
 //! - Domain types: [`crate::domain`] - Types to be wrapped in DTOs
+
+/// Data Transfer Objects (DTOs) for API responses.
+///
+/// This module contains DTO structures that are used to serialize domain
+/// objects for API responses, providing a clean separation between internal
+/// domain models and external API contracts.
+pub mod dto;
+
+pub mod error;