/// Environment configuration for the application.
///
/// Loads required configuration from environment variables at startup.
/// All variables must be present or the application will panic during [`Env::load`].
/// Used by [`AppState`](crate::AppState) for configuring JWT signing and CORS.
///
/// # Fields
/// - `db_url`: PostgreSQL database connection URL
/// - `token_secret`: Secret key used to sign and verify [`Claims`](crate::models::Claims) in JWT tokens
/// - `origin`: Frontend origin URL for CORS policy configuration
///
/// # Required Environment Variables
/// - `DATABASE_URL`: PostgreSQL connection string (e.g., `postgresql://user:pass@localhost/dbname`)
/// - `TOKEN_SECRET`: Secret key for JWT token signing (use a strong random string in production)
/// - `ORIGIN`: Frontend URL for CORS (e.g., `http://localhost:8080`)
#[derive(Debug, Clone)]
pub struct Env {
    /// PostgreSQL database connection URL
    pub db_url: String,
    /// Secret key used to sign and verify JWT tokens
    pub token_secret: String,
    /// Frontend origin URL for CORS policy
    pub origin: String,
    /// Backend port number
    pub backend_port: String,
}

impl Env {
    /// Loads environment configuration from system environment variables.
    ///
    /// Reads `DATABASE_URL`, `TOKEN_SECRET`, and `ORIGIN` from the environment and returns
    /// a configured [`Env`] instance. Used during server initialization in the `main` function.
    ///
    /// # Panics
    /// If any required variable is missing (DATABASE_URL, TOKEN_SECRET, or ORIGIN).
    ///
    /// # Example
    /// ```ignore
    /// let env = Env::load();
    /// // Environment must have DATABASE_URL, TOKEN_SECRET, and ORIGIN set
    /// let app_state = AppState {
    ///     db: pool,
    ///     env,
    /// };
    /// ```
    pub fn load() -> Env {
        let db_url = std::env::var("DATABASE_URL").expect("DATABASE_URL must be set");
        let token_secret = std::env::var("TOKEN_SECRET").expect("TOKEN_SECRET must be set");
        let origin = std::env::var("ORIGIN").expect("ORIGIN must be set");
        let backend_port = std::env::var("BACKEND_PORT").expect("BACKEND_PORT must be set");
        Env {
            db_url,
            token_secret,
            origin,
            backend_port,
        }
    }
}