D5: empty VTT captions fall back to whisper transcription

When youtube-transcript fetchTranscript() returns empty segments array
(parse succeeded but zero captions), treat as fallback trigger instead of
returning empty transcript.

- tryGetCaptions(): return CaptionResult with empty segments (not null)
  when segments array is empty. Distinguishes 'no captions available'
  (null) from 'captions exist but empty' (CaptionResult with [])
- downloadYouTube(): check captions.segments.length > 0 before taking
  fast path. Empty segments fall through to audio download + whisper
- Tests: empty array fallback, whitespace-only entries fallback,
  non-empty segments still use fast path

src/lib/server/downloader.ts

@@ -53,8 +53,6 @@ async function tryGetCaptions(url: string, _outDir: string): Promise<CaptionResu
 	try {
 		const transcript = await fetchTranscript(url, { lang: 'en' });
 		const segments = transcriptEntriesToSegments(transcript);
-		if (segments.length === 0) return null;
-
 		const title = await getYouTubeTitle(url);
 		return { type: 'captions', segments, title };
 	} catch {
@@ -109,7 +107,7 @@ export async function downloadYouTube(url: string, jobId: string): Promise<Downl
 	await mkdir(outDir, { recursive: true });
 
 	const captions = await tryGetCaptions(url, outDir);
-	if (captions) return captions;
+	if (captions && captions.segments.length > 0) return captions;
 
 	const { audioPath, title } = await downloadAudio(url, outDir);
 	return { type: 'audio', audioPath, title };