Give /v1/e/ ETag, 304 and HEAD as /v1/image/ has (closes #84)

The ETag and If-None-Match code of the /v1/image/ handler becomes
notModified, which both image handlers call; /v1/e/ answers HEAD with
the headers only and is routed for HEAD. HandleImageEnc was at the
80-line function limit, so its token checks move unchanged into
parseImageEncRequest, as parseImageRequest does for /v1/image/. No Vary
is added: only the image routes' CORS headers depend on a request
header, and go-chi/cors already sends Vary: Origin with them.

Model: opus-5-5

internal/handlers/image.go

@@ -248,6 +248,25 @@ func cacheControl(expires time.Time) string {
 	return fmt.Sprintf("public, max-age=%d, immutable", int64(maxAge/time.Second))
 }
 
+// notModified sets the ETag header to etag and, when the request's
+// If-None-Match is that ETag, answers 304 Not Modified. It reports whether it
+// answered. An empty etag sets no header and never answers.
+func notModified(w http.ResponseWriter, r *http.Request, etag string) bool {
+	if etag == "" {
+		return false
+	}
+
+	w.Header().Set("ETag", etag)
+
+	if r.Header.Get("If-None-Match") != etag {
+		return false
+	}
+
+	w.WriteHeader(http.StatusNotModified)
+
+	return true
+}
+
 // writeImageResponse writes headers and streams the image content,
 // handling conditional and HEAD requests.
 func (s *Handlers) writeImageResponse(
@@ -266,17 +285,8 @@ func (s *Handlers) writeImageResponse(
 	w.Header().Set("Cache-Control", cacheControl(req.Expires))
 	w.Header().Set("X-Pixa-Cache", string(resp.CacheStatus))
 
-	if resp.ETag != "" {
-		w.Header().Set("ETag", resp.ETag)
-
-		// Check for conditional request (If-None-Match)
-		if ifNoneMatch := r.Header.Get("If-None-Match"); ifNoneMatch != "" {
-			if ifNoneMatch == resp.ETag {
-				w.WriteHeader(http.StatusNotModified)
-
-				return
-			}
-		}
+	if notModified(w, r, resp.ETag) {
+		return
 	}
 
 	// Handle HEAD request - return headers only