v0.20.4

Added

• Implemented FromStr support for AppCode together with a lightweight
  ParseAppCodeError so RFC7807 responses and documentation examples can parse
  machine codes without bespoke helpers.

Fixed

• Removed the redundant #![cfg(feature = "axum")] attribute and tightened
  Axum, SQLx and Tonic integration tests to satisfy -D warnings builds.
• Allowed attaching JSON details via ErrorResponse::with_details without
  tripping Clippy's result_large_err lint by documenting the intentional
  AppError return shape.