Behaviour-preserving swap: rest.RealIP sets r.RemoteAddr from
X-Real-IP / X-Forwarded-For like chi's middleware.RealIP, removing chi from
the RealIP path without changing the trust model (GHSA-56x6-q882-mf27 stays
present, to be fixed separately). chi/middleware stays imported for Timeout;
whichever of this PR and the Timeout PR (#2097) merges last drops the import.
Drop-in to a tested rest middleware; covered by existing api router tests.