refactor(api): literal-union the closed-value string fields (R7 S3a·1)

Video.status, Playlist.kind/source (+ PlaylistDetail), DownloadJob.job_kind
were bare `string`; type them as named unions verified against the backend
(VALID_STATES={new,watched,hidden}; kind user|watch_later; source local|
youtube; job_kind download|edit). VideoStatus then propagates through the
optimistic-override map, onState, and the VideoCard/VirtualFeed/PlayerModal
props — tsc now rejects a status typo (e.g. a stale "in_progress", which is a
derived filter, never a stored status) at every call site.

frontend/src/components/PlayerModal.tsx

@@ -29,7 +29,7 @@ import Avatar from "./Avatar";
 import AddToPlaylist from "./AddToPlaylist";
 import { modalCount } from "./Modal";
 import DownloadButton from "./DownloadButton";
-import { api, type Video } from "../lib/api";
+import { api, type Video, type VideoStatus } from "../lib/api";
 import {
   channelYouTubeUrl,
   formatDate,
@@ -118,7 +118,7 @@ export default function PlayerModal({
   queue?: Video[];
   startIndex?: number;
   onClose: () => void;
-  onState: (id: string, status: string) => void;
+  onState: (id: string, status: VideoStatus) => void;
   // Open the active video's channel page in-app (closes the player first). The small external
   // icon next to the name keeps the open-on-YouTube behaviour.
 }) {