/**
* GET /api/docs — Rendered API reference (Redoc UI).
*
* Serves an HTML page that loads Redoc from a CDN and points it at
* `/openapi.yaml` (the same canonical spec maintained at
* `docs/openapi.yaml` in the repo). The CDN dependency is documented in
* `docs/architecture/standalone-renderer-strategy.md`; if the deployment
* is air-gapped, mirror the Redoc assets under `public/vendor/redoc/`
* and update the