fix: stage zip downloads to temp file and improve file serving headers

Write zip to a temporary file before serving to enable correct content-length, range requests, and proper cache-control headers. Additionally, handle negative object sizes by falling back to file metadata for content-length.

backend/libs/middleware/gzip.go

@@ -60,6 +60,9 @@ func shouldSkipGzip(r *http.Request) bool {
 	}
 
 	path := r.URL.Path
+	if strings.HasPrefix(path, "/d/") && (strings.HasSuffix(path, "/zip") || strings.HasSuffix(path, "/download")) {
+		return true
+	}
 	switch ext := strings.ToLower(path[strings.LastIndex(path, ".")+1:]); ext {
 	case "br", "gz", "zip", "7z", "rar", "jpg", "jpeg", "png", "gif", "webp", "avif", "mp4", "webm", "mov", "m4v", "mp3", "ogg", "woff", "woff2", "ttf", "otf":
 		return true