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

specs/001-modbus-relay-control/tasks.org

@@ -616,14 +616,16 @@ CLOSED: [2026-01-23 ven. 20:42]
   - *File*: =src/application/use_cases/get_all_relays.rs=
   - *Complexity*: Low | *Uncertainty*: Low
 
-*** STARTED Presentation Layer (Backend API) [0/2]
+*** DONE Presentation Layer (Backend API) [2/2]
+CLOSED: [2026-03-01 dim. 11:07]
+- State "DONE"       from "STARTED"    [2026-03-01 dim. 11:07]
 - State "STARTED"    from "TODO"       [2026-01-23 ven. 20:42]
-- [ ] *T045* [US1] [TDD] Define =RelayDto= in presentation layer
+- [X] *T045* [US1] [TDD] Define =RelayDto= in presentation layer
   - Fields: =id= (=u8=), =state= ("on"/"off"), =label= (=Option=)
   - Implement =From= for =RelayDto=
   - *File*: =src/presentation/dto/relay_dto.rs=
   - *Complexity*: Low | *Uncertainty*: Low
-- [ ] *T046* [US1] [TDD] Define API error responses
+- [X] *T046* [US1] [TDD] Define API error responses
   - =ApiError= enum with status codes and messages
   - Implement =poem::error::ResponseError=
   - *File*: =src/presentation/error.rs=