feat(application): implement US1 relay control use cases

Add GetAllRelaysUseCase (T043) for retrieving all 8 relay states with
labels, coordinating controller reads and repository label lookups
with comprehensive error handling and logging.

Implement ToggleRelayUseCase (T041) for toggling individual relay
states with read-before-write pattern, state validation, and label
retrieval.

Add use_cases module (T044) with trait-based dependency injection for
testability, exposing both use cases for presentation layer
integration.

Comprehensive test coverage includes 7 toggle tests (state
transitions, error handling, double-toggle idempotency) and 9 get-all
tests (count, ordering, state correctness, label inclusion, error
scenarios).

Ref: T041 T042 T043 T044 (specs/001-modbus-relay-control/tasks.org)

backend/src/application/mod.rs

@@ -65,3 +65,4 @@
 //! - Domain types: [`crate::domain`] - Domain entities and value objects
 
 pub mod health;
+pub mod use_cases;