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/controler.rs

@@ -0,0 +1,7 @@
+/// Errors that can occur during relay controller operations.
+#[derive(Debug, thiserror::Error)]
+pub enum ControllerError {
+    /// The provided relay ID is outside the valid range (1-8).
+    #[error("Invalid relay ID: {0}")]
+    InvalidRelayId(u8),
+}

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;

backend/src/domain/relay/types/mod.rs

@@ -0,0 +1,2 @@
+mod relayid;
+pub use relayid::RelayId;

backend/src/domain/relay/types/relayid.rs

@@ -1,3 +1,5 @@
+use crate::domain::relay::controler::ControllerError;
+
 /// Unique identifier for a relay in the system.
 ///
 /// Uses the newtype pattern to provide type safety and prevent mixing relay IDs
@@ -7,6 +9,33 @@
 #[repr(transparent)]
 pub struct RelayId(u8);
 
+impl RelayId {
+    /// Creates a new `RelayId` from a relay channel number.
+    ///
+    /// This is a smart constructor that validates the relay ID is within the
+    /// valid range (1-8) for an 8-channel relay controller.
+    ///
+    /// # Errors
+    ///
+    /// Returns `ControllerError::InvalidRelayId` if the ID is not in the range 1-8.
+    pub const fn new(id: u8) -> Result<Self, ControllerError> {
+        if id > 0 && id < 9 {
+            Ok(Self(id))
+        } else {
+            Err(ControllerError::InvalidRelayId(id))
+        }
+    }
+
+    /// Returns the inner `u8` value of the relay ID.
+    ///
+    /// This accessor method provides access to the underlying relay channel number.
+    #[must_use]
+    pub const fn as_u8(&self) -> u8 {
+        self.0
+    }
+
+}
+
 impl std::fmt::Display for RelayId {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         self.0.fmt(f)

specs/001-modbus-relay-control/tasks.md

@@ -211,7 +211,7 @@
   - **File**: src/domain/relay.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T018** [US1] [TDD] Implement RelayId newtype with validation
+- [x] **T018** [US1] [TDD] Implement RelayId newtype with validation
   - #[repr(transparent)] newtype wrapping u8
   - Constructor validates 1..=8 range
   - Implement Display, Debug, Clone, Copy, PartialEq, Eq