//! # config — one schema, three sources
//!
//! Configuration resolves in precedence order: CLI flags, then
//! `MAILBOURNE_*` environment variables, then `/var/mailbourne/
//! mailbourne.toml`, then defaults. Every TOML key has an env twin so
//! containers never need file templating. The generated TOML is heavily
//! commented — the config file doubles as a guided tour.
//!
//! The schema lands here as the engine grows; it starts with the one value
//! nothing can work without.
/// Engine configuration.