v0.15.0

Added

• Introduced a response::problem_json module with an RFC7807 ProblemJson
  payload that serializes metadata, gRPC mappings and retry/authentication
  hints while respecting the message redaction policy.
• Added an optional tonic feature exposing TryFrom<Error> for tonic::Status
  with sanitized metadata and canonical gRPC code mapping.
• Published a compile-time CODE_MAPPINGS table mapping each AppCode to
  HTTP, gRPC and problem type information for reuse across transports.

Changed

• Updated Axum and Actix integrations to emit application/problem+json
  bodies, attach Retry-After/WWW-Authenticate headers automatically and
  avoid leaking redactable messages or metadata.
• Re-exported ProblemJson from the crate root alongside ErrorResponse for
  direct construction in custom handlers.

Tests

• Added unit coverage for the problem+json metadata sanitizer, header
  propagation in Axum, and gRPC code mapping under the new tonic feature.