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/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).