Send the request's ID back as X-Request-Id

A middleware right after chi's RequestID sets the X-Request-Id response
header from the ID RequestID stored in the request context, so every
response carries it and a client can quote it when reporting a problem.
README "Routes" says where the ID comes from.

Model: opus-5-5

internal/middleware/middleware.go

@@ -115,6 +115,20 @@ func (s *Middleware) RateLimit(
 		})
 }
 
+// RequestIDResponseHeader returns a middleware that sends the request's ID as
+// the X-Request-Id response header, so a client can quote it when reporting a
+// problem. The ID is the one chi's RequestID middleware stored in the request
+// context, so RequestID must run first.
+func (s *Middleware) RequestIDResponseHeader() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set(middleware.RequestIDHeader,
+				middleware.GetReqID(r.Context()))
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
 type loggingResponseWriter struct {
 	http.ResponseWriter