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/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;