v0.17.0

Added

• Per-field redaction metadata via a new [FieldRedaction] enum, default
  heuristics for common secret keys (passwords, tokens, card numbers) and the
  Context::redact_field / AppError::redact_field helpers.
• #[masterror(redact(fields(...)))] support in the derive macro to configure
  metadata policies alongside message redaction.
• Opt-in internal formatters for [ErrorResponse] and [ProblemJson] that are
  safe to use in diagnostic logs without additional serialization boilerplate.

Changed

• Problem JSON and legacy ErrorResponse serialization now hash, mask or drop
  metadata according to per-field policies while honoring the global redaction
  flag.
• Redaction-aware conversions ensure redactable messages fall back to the error
  kind across HTTP and gRPC mappings.