fix(transcript): collapse rolling segment echoes

Normalize incremental backend hypothesis chains before persistence and ignore stale or replayed webhook callbacks so duplicate transcript text does not survive ingest.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/tests/webhook.test.ts

@@ -132,6 +132,43 @@ describe('POST /api/webhook/[jobId] — locally cancelled job', () => {
 	});
 });
 
+// ── Duplicate / stale callback guards ──────────────────────────────────────────
+
+describe('POST /api/webhook/[jobId] — duplicate and stale callbacks', () => {
+	it('ignores replayed success callbacks after the transcript is already done', async () => {
+		mockGetJob.mockReturnValue({
+			...makeJob('job-done'),
+			status: 'done',
+			segmentsJson: JSON.stringify([makeSeg(0, 'Already saved.')]),
+			whisperJobId: 'whisper-id'
+		});
+
+		const res = await POST(makeEvent('job-done', makeWhisperJob()) as any);
+		expect(res.status).toBe(200);
+		expect(await res.json()).toEqual({ ok: true, ignored: 'duplicate_webhook' });
+		expect(mockSetJobStatus).not.toHaveBeenCalled();
+		expect(mockUpdateJob).not.toHaveBeenCalled();
+		expect(mockWriteOutputs).not.toHaveBeenCalled();
+	});
+
+	it('ignores stale callbacks from an older whisper job after retry', async () => {
+		mockGetJob.mockReturnValue({
+			...makeJob('job-stale'),
+			status: 'transcribing',
+			whisperJobId: 'current-whisper-job'
+		});
+
+		const res = await POST(
+			makeEvent('job-stale', makeWhisperJob({ id: 'old-whisper-job', segments: [makeSeg(0, 'stale')] })) as any
+		);
+		expect(res.status).toBe(200);
+		expect(await res.json()).toEqual({ ok: true, ignored: 'stale_whisper_job' });
+		expect(mockSetJobStatus).not.toHaveBeenCalled();
+		expect(mockUpdateJob).not.toHaveBeenCalled();
+		expect(mockWriteOutputs).not.toHaveBeenCalled();
+	});
+});
+
 // ── Whisper job failed / cancelled ───────────────────────────────────────────
 
 describe('POST /api/webhook/[jobId] — whisper failure', () => {