It appears that the repository has undergone several changes and renamings:

backend/node_modules/date-fns/locale/de-AT.js

@@ -1,30 +0,0 @@
-import { formatDistance } from "./de/_lib/formatDistance.js";
-import { formatLong } from "./de/_lib/formatLong.js";
-import { formatRelative } from "./de/_lib/formatRelative.js";
-import { match } from "./de/_lib/match.js";
-
-// difference to 'de' locale
-import { localize } from "./de-AT/_lib/localize.js";
-
-/**
- * @category Locales
- * @summary German locale (Austria).
- * @language German
- * @iso-639-2 deu
- * @author Christoph Tobias Stenglein [@cstenglein](https://github.com/cstenglein)
- */
-export const deAT = {
-  code: "de-AT",
-  formatDistance: formatDistance,
-  formatLong: formatLong,
-  formatRelative: formatRelative,
-  localize: localize,
-  match: match,
-  options: {
-    weekStartsOn: 1 /* Monday */,
-    firstWeekContainsDate: 4,
-  },
-};
-
-// Fallback for modularized imports:
-export default deAT;