feat(domain): implement RelayState enum with serialization support

Add RelayState enum to domain layer with:
- Display, Debug, Clone, Copy, PartialEq, Eq derives
- serde Serialize/Deserialize traits for API JSON handling
- Type-safe representation of relay on/off states

TDD green phase: Tests from T019 now pass.

Ref: T020 (specs/001-modbus-relay-control/tasks.md)

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

@@ -1,11 +1,13 @@
 /// State of a relay (on or off).
 ///
 /// Represents the binary state of a relay channel in the Modbus controller.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum RelayState {
     /// Relay is energized (circuit closed).
+    #[serde(rename = "on")]
     On,
     /// Relay is de-energized (circuit open).
+    #[serde(rename = "off")]
     Off,
 }