feat: VTT fallback to whisper with transcript_source tracking

- Set transcript_source to 'captions' when using VTT captions (caption fast path)
- Set transcript_source to 'whisper' when whisper transcription completes via webhook
- Add comprehensive test suite for VTT fallback scenarios
- Tests verify empty/whitespace captions trigger whisper, non-empty captions skip whisper
- Persists transcript_source to database for audit trail

Closes story-d5-vtt-fallback-y-d1d6

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/routes/api/webhook/[jobId]/+server.ts

@@ -72,7 +72,8 @@ export async function POST({ params, request }) {
 			status: 'done',
 			progress: 100,
 			segmentsJson: JSON.stringify(segments),
-			outputDir
+			outputDir,
+			transcriptSource: 'whisper'
 		});
 
 		emitProgress(jobId, { type: 'done', status: 'done' });