Send the request ID upstream and log it with each image

The fetcher sends the request ID chi's RequestID middleware put in the
context as X-Request-Id, next to User-Agent and Accept, and the
"upstream fetched", "image converted" and "image served" lines carry it
as request_id, as the request log line does. A fetch shared by several
requests runs with the first request's context values, so it carries
that request's ID; README says so where it describes the shared fetch.

Model: opus-5-5

internal/httpfetcher/httpfetcher.go

@@ -18,6 +18,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"github.com/go-chi/chi/v5/middleware"
 )
 
 // Fetcher configuration constants.
@@ -267,6 +269,13 @@ func (f *HTTPFetcher) Fetch(ctx context.Context, url string) (*FetchResult, erro
 	req.Header.Set("User-Agent", f.config.UserAgent)
 	req.Header.Set("Accept", strings.Join(f.config.AllowedContentTypes, ", "))
 
+	// The ID of the request this fetch serves, so the fetch can be found in
+	// the upstream host's logs
+	requestID := middleware.GetReqID(ctx)
+	if requestID != "" {
+		req.Header.Set(middleware.RequestIDHeader, requestID)
+	}
+
 	// Use httptrace to capture connection details
 	var remoteAddr string