feat(domain): implement RelayId newtype with validation

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)

backend/src/domain/relay/mod.rs

@@ -7,3 +7,5 @@
 pub mod repository;
 /// Domain types for relay identification and control.
 pub mod types;
+/// Controller error types for relay operations.
+pub mod controler;