Replace chi middleware.Throttle with rest.Throttle on the main router

Drop-in swap of the global concurrency limiter (go-pkgz/rest.Throttle has
the same signature and semantics as chi's middleware.Throttle), with the
chi router left in place. First of the per-middleware swaps that chip away
at go-chi/chi/v5/middleware before the router itself is migrated.

chi/middleware is still imported for RealIP/Timeout/NoCache; build, vet,
race tests and golangci-lint clean.

backend/app/rest/api/rest.go

@@ -221,7 +221,7 @@ func (s *Rest) routes() chi.Router {
 		s.openRouteLimiter = openRouteLimiter
 	}
 	router := chi.NewRouter()
-	router.Use(middleware.Throttle(1000), middleware.RealIP, R.Recoverer(log.Default()))
+	router.Use(R.Throttle(1000), middleware.RealIP, R.Recoverer(log.Default()))
 	router.Use(securityHeadersMiddleware(s.ExternalImageProxy, s.AllowedAncestors))
 	if !s.DisableSignature {
 		router.Use(R.AppInfo("remark42", "umputun", s.Version))