feat(settings): add CorsSettings struct for CORS configuration

Implements CorsSettings struct with validation and deserialization support
for configuring Cross-Origin Resource Sharing in the application settings.

Ref: T010 (specs/001-modbus-relay-control)

backend/src/startup.rs

@@ -181,8 +181,7 @@ mod tests {
                 burst_size: 100,
                 per_seconds: 60,
             },
-            modbus: crate::settings::ModbusSettings::default(),
-            relay: crate::settings::RelaySettings::default(),
+            ..Default::default()
         }
     }