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/Feed.tsx

@@ -22,7 +22,7 @@ import {
   Youtube,
   type LucideIcon,
 } from "lucide-react";
-import { api, HttpError, type FeedFilters, type Video } from "../lib/api";
+import { api, HttpError, type FeedFilters, type Video, type VideoStatus } from "../lib/api";
 import i18n from "../i18n";
 import { notify, resolveVideo } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
@@ -70,7 +70,7 @@ const VIEW_ICON: Record<FeedView, LucideIcon> = {
 // (which encode both). One entry per concept; a single arrow flips the direction.
 // "relevance" (full-text search ranking) and "shuffle" are directionless single-string sorts.
 
-function matchesView(status: string, show: string): boolean {
+function matchesView(status: VideoStatus, show: string): boolean {
   switch (show) {
     case "hidden":
       return status === "hidden";
@@ -126,7 +126,7 @@ export default function Feed({
   const sortKeys = channelScoped
     ? SORT_KEYS.filter((k) => k !== "subscribers" && k !== "priority")
     : SORT_KEYS;
-  const [overrides, setOverrides] = useState<Record<string, string>>({});
+  const [overrides, setOverrides] = useState<Record<string, VideoStatus>>({});
   const [savedOverrides, setSavedOverrides] = useState<Record<string, boolean>>({});
   // The open player: which video and where to start (null = resume from saved position).
   const [activeVideo, setActiveVideo] = useState<{ video: Video; startAt: number | null } | null>(
@@ -241,7 +241,7 @@ export default function Feed({
   const loadedRef = useRef<Video[]>([]);
 
   const onState = useCallback(
-    (id: string, status: string) => {
+    (id: string, status: VideoStatus) => {
       setOverrides((o) => ({ ...o, [id]: status }));
       // Refetch once the server has the change so other views (e.g. Hidden) are in sync.
       // Announce the change only AFTER the server confirms it: a "Marked watched"/"Hidden"