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/settings/base.yaml

@@ -8,7 +8,7 @@ rate_limit:
   per_seconds: 60
 
 modbus:
-  host: "192.168.0.200"
+  host: 192.168.0.200
   port: 502
   slave_id: 0
   timeout_secs: 5