Add numeric business codes, include `trace_id`, and support custom
error messages and `details` for error responses while keeping envelope
wrapping idempotent across old and new formats.
BREAKING CHANGE: response envelope `code` changes from string to int and
envelope format now includes `trace_id` (and may include `details`).
Add response envelope middleware to standardize JSON responses as
`{code,data,message}` with consistent business codes across endpoints.
Update Swagger annotations and tests to reflect the new response shape.
BREAKING CHANGE: API responses are now wrapped in a response envelope; clients must read payloads from `data` and handle `code`/`message` fields.
