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/handlers/image.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
@@ -298,6 +299,7 @@ func (s *Handlers) writeImageResponse(
 	// Log cache status and timing after serving
 	duration := time.Since(startTime)
 	s.log.Info("image served",
+		"request_id", middleware.GetReqID(r.Context()),
 		"cache_key", cacheKey,
 		"cache_status", resp.CacheStatus,
 		"duration_ms", duration.Milliseconds(),