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 };

src/tests/downloader.test.ts

@@ -133,3 +133,76 @@ describe('downloadYouTube', () => {
 		});
 	});
 });
+
+describe('empty captions fallback (D5)', () => {
+	it('falls back to audio when fetchTranscript returns empty segments array', async () => {
+		mockFetchTranscript.mockResolvedValue([] satisfies TranscriptResponse[]);
+		mockExecFile.mockImplementation((...args: unknown[]) => {
+			const allArgs = args.slice(0, -1) as string[];
+			const cb = args.at(-1) as (...callbackArgs: unknown[]) => void;
+			// downloadAudio: yt-dlp -f bestaudio [...]
+			if (allArgs.includes('-f')) {
+				const oIdx = allArgs.indexOf('-o');
+				const outPattern = oIdx !== -1 ? allArgs[oIdx + 1] : '';
+				const outDir = outPattern ? outPattern.substring(0, outPattern.lastIndexOf('/')) : '';
+				if (outDir) {
+					const { mkdirSync, writeFileSync } = require('fs');
+					mkdirSync(outDir, { recursive: true });
+					writeFileSync(require('path').join(outDir, 'audio.mp4'), 'fake-audio');
+					writeFileSync(require('path').join(outDir, 'audio.info.json'), JSON.stringify({ title: 'Empty Captions Video' }));
+				}
+			}
+			cb(null, JSON.stringify({ title: 'Empty Captions Video' }), '');
+		});
+
+		const result = await downloadYouTube('https://youtube.com/watch?v=empty', 'test-empty-1');
+
+		expect(mockFetchTranscript).toHaveBeenCalledWith('https://youtube.com/watch?v=empty', {
+			lang: 'en'
+		});
+		expect(result.type).toBe('audio');
+		expect(result.title).toBe('Empty Captions Video');
+	});
+
+	it('falls back to audio when all transcript entries are whitespace', async () => {
+		mockFetchTranscript.mockResolvedValue([
+			{ text: '   ', offset: 1000, duration: 2000, lang: 'en' },
+			{ text: ' ', offset: 3000, duration: 1000, lang: 'en' }
+		] satisfies TranscriptResponse[]);
+		mockExecFile.mockImplementation((...args: unknown[]) => {
+			const allArgs = args.slice(0, -1) as string[];
+			const cb = args.at(-1) as (...callbackArgs: unknown[]) => void;
+			if (allArgs.includes('-f')) {
+				const oIdx = allArgs.indexOf('-o');
+				const outPattern = oIdx !== -1 ? allArgs[oIdx + 1] : '';
+				const outDir = outPattern ? outPattern.substring(0, outPattern.lastIndexOf('/')) : '';
+				if (outDir) {
+					const { mkdirSync, writeFileSync } = require('fs');
+					mkdirSync(outDir, { recursive: true });
+					writeFileSync(require('path').join(outDir, 'audio.mp4'), 'fake-audio');
+					writeFileSync(require('path').join(outDir, 'audio.info.json'), JSON.stringify({ title: 'Whitespace Only' }));
+				}
+			}
+			cb(null, JSON.stringify({ title: 'Whitespace Only' }), '');
+		});
+
+		const result = await downloadYouTube('https://youtube.com/watch?v=ws-only', 'test-empty-2');
+
+		expect(result.type).toBe('audio');
+	});
+
+	it('still uses caption fast path when fetchTranscript returns non-empty segments', async () => {
+		mockFetchTranscript.mockResolvedValue([
+			{ text: 'Real caption', offset: 1000, duration: 2000, lang: 'en' }
+		] satisfies TranscriptResponse[]);
+		mockExecFile.mockImplementation((...args: unknown[]) => {
+			const cb = args.at(-1) as (...callbackArgs: unknown[]) => void;
+			cb(null, JSON.stringify({ title: 'Real Caption Video' }), '');
+		});
+
+		const result = await downloadYouTube('https://youtube.com/watch?v=has-captions', 'test-has-cap');
+
+		expect(result.type).toBe('captions');
+		expect((result as { type: 'captions'; segments: unknown[] }).segments.length).toBe(1);
+	});
+});