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.
Replace health_handler with status_handler providing public /status and
/about endpoints. Add build-time version injection via ldflags in
Makefile, and support --version/-v CLI flag.
- Add /status endpoint returning runtime status, uptime, and version
- Add /about endpoint with system metadata (name, description, repo)
- Configure VERSION variable with git describe fallback
- Update swagger docs and api.md for new public endpoints
- Remove deprecated /api/status/test endpoint
