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/lib/api.ts

@@ -47,6 +47,11 @@ export interface Tag {
   channel_count: number;
 }
 
+// Per-user watch status stored server-side (VALID_STATES in routes/feed.py). "in_progress" and
+// "unwatched" are NOT statuses — they're derived feed filters (status "new" + a resume position),
+// so they never appear on the wire.
+export type VideoStatus = "new" | "watched" | "hidden";
+
 export interface Video {
   id: string;
   title: string | null;
@@ -60,7 +65,7 @@ export interface Video {
   view_count: number | null;
   is_short: boolean;
   live_status: string;
-  status: string;
+  status: VideoStatus;
   position_seconds: number;
   saved: boolean; // is the video in the user's built-in Watch later playlist
   watch_url: string;
@@ -124,11 +129,14 @@ export interface FeedResponse {
   source?: "scrape" | "api";
 }
 
+export type PlaylistKind = "user" | "watch_later";
+export type PlaylistSource = "local" | "youtube";
+
 export interface Playlist {
   id: number;
   name: string;
-  kind: string; // "user" | "watch_later"
-  source: string; // "local" | "youtube"
+  kind: PlaylistKind;
+  source: PlaylistSource;
   yt_playlist_id: string | null;
   dirty: boolean; // linked local playlist has edits not yet pushed to YouTube
   item_count: number;
@@ -140,8 +148,8 @@ export interface Playlist {
 export interface PlaylistDetail {
   id: number;
   name: string;
-  kind: string;
-  source: string;
+  kind: PlaylistKind;
+  source: PlaylistSource;
   yt_playlist_id: string | null;
   dirty: boolean;
   items: Video[];
@@ -1012,6 +1020,8 @@ interface DownloadAsset {
 }
 
 export type DownloadStatus = "queued" | "running" | "paused" | "done" | "error" | "canceled";
+// "download" (default) or "edit" (a per-user trim/crop derivative cut from another job).
+export type DownloadJobKind = "download" | "edit";
 
 // Phase-2 editor recipe. A single `trim` (one output file) or a `segments` cut-list joined into
 // one file. `accurate` re-encodes for a frame-accurate cut (a crop always re-encodes).
@@ -1051,7 +1061,7 @@ export interface DownloadJob {
   display_uploader: string | null; // user override of the channel name
   display_uploader_url: string | null; // user override of the channel link
   extra_links: string[]; // extra reference URLs (beyond source_url)
-  job_kind?: string; // "download" (default) | "edit"
+  job_kind?: DownloadJobKind; // "download" (default) | "edit" (a per-user trim/crop derivative)
   source_job_id?: number | null; // parent download an edit was cut from
   edit_spec?: EditSpec | null;
   can_download: boolean;