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

README.md

@@ -137,6 +137,12 @@ path under `/v1/` answers 200, in maintenance mode too.
   authentication with `metrics.username` and `metrics.password`. Answers: 200;
   401 without them; 404 when they are not set, as the route then does not exist.
 
+Every response carries an `X-Request-ID` header holding the request's ID, which
+a client can quote when reporting a problem: the request's own `X-Request-ID`
+when it sent one, as a reverse proxy in front of pixa may, otherwise one pixa
+makes up from its host name, a random string chosen at startup and a counter.
+pixa's log line for the request carries the same ID as `request_id`.
+
 Both `POST` routes accept only a form that pixa's own page served: the page puts
 a token in the form and sets a cookie to match, and a request without both is
 refused with 403, so another site cannot submit the form from a visitor's

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
 

internal/server/routes.go

@@ -29,6 +29,7 @@ func (s *Server) SetupRoutes() {
 
 	s.router.Use(middleware.Recoverer)
 	s.router.Use(middleware.RequestID)
+	s.router.Use(s.mw.RequestIDResponseHeader())
 	s.router.Use(s.mw.ClientIP())
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())