Return and pass on request IDs, and give /v1/e/ ETag, 304 and HEAD (closes #84)

Every response carries X-Request-Id, the upstream fetch sends it, and
the "upstream fetched", "image converted" and "image served" lines log
it as request_id. pixa's own RequestID middleware keeps a request's own
ID only when it is at most 64 letters, digits, '-', '_' or '.', and
otherwise makes a random one with crypto/rand, so nothing a client
chooses freely and nothing about the host reaches upstream. /v1/e/ now
sets ETag, answers a matching If-None-Match with 304 and is routed for
HEAD, through notModified, which both image handlers call. No Vary is
added: go-chi/cors already sends Vary: Origin.

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