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/VirtualFeed.tsx

@@ -1,4 +1,4 @@
-import type { Video } from "../lib/api";
+import type { Video, VideoStatus } from "../lib/api";
 import { FEED_VIEW_SPEC, type FeedView } from "../lib/feedView";
 import VideoCard from "./VideoCard";
 import VirtualGrid from "./VirtualGrid";
@@ -21,7 +21,7 @@ const ROW_EST: Record<FeedView, number> = {
 export interface VirtualFeedProps {
   items: Video[];
   view: FeedView;
-  onState: (id: string, status: string) => void;
+  onState: (id: string, status: VideoStatus) => void;
   onToggleSave: (id: string, saved: boolean) => void;
   onResetState: (id: string) => void;
   onOpen: (v: Video, startAt?: number | null) => void;