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/Cargo.toml

@@ -24,6 +24,7 @@ poem = { version = "3.1.12", default-features = false, features = ["csrf", "rust
 poem-openapi = { version = "5.1.16", features = ["chrono", "swagger-ui"] }
 serde = "1.0.228"
 serde_json = "1.0.148"
+serde_yaml = "0.9.34"
 sqlx = { version = "0.8.6", features = ["runtime-tokio", "sqlite", "derive", "migrate"] }
 thiserror = "2.0.17"
 tokio = { version = "1.48.0", features = ["macros", "rt-multi-thread"] }