Add complete SQLite implementation of RelayLabelRepository trait with
all CRUD operations (get_label, save_label, delete_label, get_all_labels).
Key changes:
- Create infrastructure entities module with RelayLabelRecord struct
- Implement TryFrom traits for converting between database records and domain types
- Add From<sqlx::Error> and From<RelayLabelError> for RepositoryError
- Write comprehensive functional tests covering all repository operations
- Verify proper handling of edge cases (empty results, overwrites, max length)
TDD phase: GREEN - All repository trait tests now passing with SQLite implementation
Ref: T036 (specs/001-modbus-relay-control/tasks.md)
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)
