OpenAPI / Swagger UI (Middlewares — P2)

  - **What:** Serve generated OpenAPI docs & UI; conventions for grouping tags and describing errors.
  - **Why Middlewares:** External tooling and preferences differ.
  - **Precedents:** `swaggo` for Gin and Echo; Fiber swagger contrib. ([GitHub](https://github.com/swaggo/gin-swagger)], [docs.gofiber.io](https://docs.gofiber.io/contrib/swagger/)])
