feat(application): HealthMonitor service and hardware integration test

Add HealthMonitor service for tracking system health status with
comprehensive state transition logic and thread-safe operations.
Includes 16 unit tests covering all functionality including concurrent
access scenarios.

Add optional Modbus hardware integration tests with 7 test cases for
real device testing. Tests are marked as ignored and can be run with

Ref: T034, T039, T040 (specs/001-modbus-relay-control/tasks.org)

backend/src/application/mod.rs

@@ -11,6 +11,11 @@
 //! - **Use case driven**: Each module represents a specific business use case
 //! - **Testable in isolation**: Can be tested with mock infrastructure implementations
 //!
+//! # Submodules
+//!
+//! - `health`: Health monitoring service
+//!   - `health_monitor`: Tracks system health status and state transitions
+//!
 //! # Planned Submodules
 //!
 //! - `relay`: Relay control use cases
@@ -58,3 +63,5 @@
 //! - Architecture: `specs/constitution.md` - Hexagonal Architecture principles
 //! - Use cases: `specs/001-modbus-relay-control/plan.md` - Implementation plan
 //! - Domain types: [`crate::domain`] - Domain entities and value objects
+
+pub mod health;