fix(api): un-export types used only within their own module (knip gate)

The S3b api.ts split introduced 5 exported-but-never-imported types
(PlaylistKind/PlaylistSource, DownloadJobKind, YT internal option/data shapes).
The publish gate's knip lane flagged them (my per-step checks ran tsc/eslint/
prettier/vitest but not knip). They're used inside their own modules by the
interfaces, so make them local (consumers compare against literals, never name
the type).

frontend/src/lib/api.ts

@@ -159,8 +159,10 @@ export interface FeedResponse {
   // or optional (dead surface). Re-add if a consumer ever needs it.
 }
 
-export type PlaylistKind = "user" | "watch_later";
-export type PlaylistSource = "local" | "youtube";
+// Not exported: consumers compare `playlist.kind`/`.source` against string literals, they never
+// name these types — keeping them local satisfies the no-unused-exports gate (knip).
+type PlaylistKind = "user" | "watch_later";
+type PlaylistSource = "local" | "youtube";
 
 export interface Playlist {
   id: number;

frontend/src/lib/api/downloads.ts

@@ -41,7 +41,8 @@ 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";
+// Local (not exported): consumers compare `job.job_kind` against literals, never name the type.
+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).

frontend/src/lib/youtube.ts

@@ -3,7 +3,7 @@
 // @types/youtube we declare just the methods/events PlayerModal calls, so tsc flags any new call
 // that isn't covered here. See [[reference-yt-iframe-api-remote-control-map]] for what's driveable.
 
-export interface YTVideoData {
+interface YTVideoData {
   title: string;
   author: string;
   video_id?: string;
@@ -34,7 +34,7 @@ export interface YTPlayerEvent {
   target: YTPlayer;
 }
 
-export interface YTPlayerOptions {
+interface YTPlayerOptions {
   width?: string | number;
   height?: string | number;
   videoId?: string;