feat: whisper-side cancellation + SSE-triggered retry

- Add cancelJob() to whisper.ts: sends DELETE /jobs/:id to the whisper
  server (best-effort, errors silently ignored)
- DELETE /api/jobs/[id] now calls cancelJob() when cancelling an active
  job that has a whisperJobId, stopping GPU use immediately
- Webhook handler guards against locally-cancelled jobs: returns ok early
  so whisper's late completion cannot overwrite cancelled status or send
  a phantom 'Transcript ready' notification
- Replace blind sleep(Retry-After + 1s) in submitJob() with
  waitForModelReady(): subscribes to /model/events SSE and proceeds as
  soon as state:ready arrives; falls back to the Retry-After timeout if
  SSE is unreachable or closes without model_ready
- Refactor retry tests to use URL-aware makeJobFetch() helper; add 7 new
  tests (3 SSE-triggered retry, 3 cancelJob, 1 webhook cancelled-guard)
  — 144/144 passing

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

src/tests/webhook.test.ts

@@ -113,6 +113,25 @@ describe('POST /api/webhook/[jobId] — job not found', () => {
 	});
 });
 
+// ── Local cancellation guard ──────────────────────────────────────────────────
+
+describe('POST /api/webhook/[jobId] — locally cancelled job', () => {
+	it('returns ok without processing when the local job is already cancelled', async () => {
+		mockGetJob.mockReturnValue({ ...makeJob('job-lc'), status: 'cancelled' });
+		const payload = makeWhisperJob({ status: 'done' });
+
+		const res = await POST(makeEvent('job-lc', payload) as any);
+		expect(res.status).toBe(200);
+		expect(await res.json()).toEqual({ ok: true });
+
+		// Must not touch outputs, status, or notifications
+		expect(mockSetJobStatus).not.toHaveBeenCalled();
+		expect(mockUpdateJob).not.toHaveBeenCalled();
+		expect(mockWriteOutputs).not.toHaveBeenCalled();
+		expect(mockSendNotification).not.toHaveBeenCalled();
+	});
+});
+
 // ── Whisper job failed / cancelled ───────────────────────────────────────────
 
 describe('POST /api/webhook/[jobId] — whisper failure', () => {