Add choices API endpoint and OpenAPI client setup

Introduces a new /api/choices/ endpoint for fetching model choices with multilingual labels. Updates Django models to use 'cz#' prefix for Czech labels. Adds OpenAPI client generation via orval, refactors frontend API structure, and provides documentation and helper scripts for dynamic choices and OpenAPI usage.

frontend/src/pages/downloader/Downloader.tsx

@@ -5,7 +5,7 @@ import {
   FORMAT_EXTS,
   type InfoResponse,
   parseContentDispositionFilename,
-} from "../../api/apps/Downloader";
+} from "../../api/legacy/Downloader";
 
 export default function Downloader() {
   const [url, setUrl] = useState("");