Unified API error format + new endpoints

Breaking: error responses are now a single shape, {"error","message"}, across every endpoint (some previously returned {"detail":...}). Update any client that parses the error body; HTTP status codes are unchanged. New /v1/convert (metal/currency/unit conversion) and /v1/carat (per-gram price by karat) endpoints. Public endpoints now cache at a cadence matched to their refresh rate. The OpenAPI spec documents error responses, and all timestamps are standardized to Z.