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/.tarpaulin.local.toml

@@ -4,4 +4,4 @@ skip-clean = true
 target-dir = "coverage"
 output-dir = "coverage"
 fail-under = 60
-exclude-files = ["target/*", "private/*", "tests/*"]
+exclude-files = ["target/*", "private/*", "backend/tests/*", "backend/build.rs"]