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;