Implement smart constructor that validates relay IDs are within valid
range (1-8 for 8-channel relay controller). Add accessor method as_u8()
for safe access to inner value. Add comprehensive documentation to satisfy
clippy requirements.
TDD green phase: Tests from T017 now pass.
Ref: T018 (specs/001-modbus-relay-control/tasks.md)
Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.
Ref: T007 (specs/001-modbus-relay-control)