Answer 499 to a client that goes away while waiting for an image (closes #339)

When a request waits for an image another request is making and its
client goes away, the image proxy returns context.Canceled. serveImage
now answers it 499, the status nginx calls "client closed request". It
logs it once at info level with the request_id, host and path. It was
answered 500 and logged at error level, as pixa's own failure. A
cancellation while the request's context is live is still answered 500.
The request making the image is still answered as before. README.md
lists the 499 on both image routes.

Model: opus-5-5

internal/handlers/client_gone_internal_test.go

@@ -9,6 +9,7 @@ import (
 	"net/http/httptest"
 	"slices"
 	"testing"
+	"time"
 
 	"github.com/go-chi/chi/v5/middleware"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
@@ -101,8 +102,10 @@ func TestImageRoutes_ClientGoneAnswers499(t *testing.T) {
 					ctx, http.MethodGet, target, nil))
 			}()
 
-			// The fetch is still held, so the second request can end only
-			// because its client went away
+			// Give the second request time to start waiting for the first
+			// request's image. The fetch is still held, so the second request
+			// can end only because its client went away
+			time.Sleep(100 * time.Millisecond)
 			cancel()
 			<-secondDone