feat: wire relay API with dependency injection

- split settings module into per-struct files
- add DatabaseSettings with default in-memory SQLite path
- implement RelayApi struct with GET /relays and POST
  /relays/{id}/toggle
- wire create_relay_controller and create_label_repository into
  Application::build() with mock/real selection via cfg!(test) || CI
- register RelayApi in OpenApiService alongside existing APIs

backend/src/settings/environment.rs

@@ -0,0 +1,134 @@
+/// Application environment.
+#[derive(Debug, PartialEq, Eq, Default)]
+pub enum Environment {
+    /// Development environment
+    #[default]
+    Development,
+    /// Production environment
+    Production,
+}
+
+impl std::fmt::Display for Environment {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let self_str = match self {
+            Self::Development => "development",
+            Self::Production => "production",
+        };
+        write!(f, "{self_str}")
+    }
+}
+
+impl TryFrom<String> for Environment {
+    type Error = String;
+
+    fn try_from(value: String) -> Result<Self, Self::Error> {
+        Self::try_from(value.as_str())
+    }
+}
+
+impl TryFrom<&str> for Environment {
+    type Error = String;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        match value.to_lowercase().as_str() {
+            "development" | "dev" => Ok(Self::Development),
+            "production" | "prod" => Ok(Self::Production),
+            other => Err(format!(
+                "{other} is not a supported environment. Use either `development` or `production`"
+            )),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn environment_display_development() {
+        let env = Environment::Development;
+        assert_eq!(env.to_string(), "development");
+    }
+
+    #[test]
+    fn environment_display_production() {
+        let env = Environment::Production;
+        assert_eq!(env.to_string(), "production");
+    }
+
+    #[test]
+    fn environment_from_str_development() {
+        assert_eq!(
+            Environment::try_from("development").unwrap(),
+            Environment::Development
+        );
+        assert_eq!(
+            Environment::try_from("dev").unwrap(),
+            Environment::Development
+        );
+        assert_eq!(
+            Environment::try_from("Development").unwrap(),
+            Environment::Development
+        );
+        assert_eq!(
+            Environment::try_from("DEV").unwrap(),
+            Environment::Development
+        );
+    }
+
+    #[test]
+    fn environment_from_str_production() {
+        assert_eq!(
+            Environment::try_from("production").unwrap(),
+            Environment::Production
+        );
+        assert_eq!(
+            Environment::try_from("prod").unwrap(),
+            Environment::Production
+        );
+        assert_eq!(
+            Environment::try_from("Production").unwrap(),
+            Environment::Production
+        );
+        assert_eq!(
+            Environment::try_from("PROD").unwrap(),
+            Environment::Production
+        );
+    }
+
+    #[test]
+    fn environment_from_str_invalid() {
+        let result = Environment::try_from("invalid");
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("not a supported environment"));
+    }
+
+    #[test]
+    fn environment_from_string_development() {
+        assert_eq!(
+            Environment::try_from("development".to_string()).unwrap(),
+            Environment::Development
+        );
+    }
+
+    #[test]
+    fn environment_from_string_production() {
+        assert_eq!(
+            Environment::try_from("production".to_string()).unwrap(),
+            Environment::Production
+        );
+    }
+
+    #[test]
+    fn environment_from_string_invalid() {
+        let result = Environment::try_from("invalid".to_string());
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn environment_default_is_development() {
+        let env = Environment::default();
+        assert_eq!(env, Environment::Development);
+    }
+
+}