fix: FormData stream exhausted on retry + undefined segments crash

Two bugs triggered together when the model was unloaded during a job:

1. submitJob() created FormData/createReadStream once outside the retry loop.
   After a 503, the audio ReadStream was consumed and subsequent retries sent
   an empty body to whisper, causing it to return segments:undefined.

2. webhook handler cast whisperJob.segments as Segment[] without guarding
   against undefined, so deduplicateSegments(undefined) crashed with
   'Cannot read properties of undefined (reading 'map')' — stored as job.error.

Fixes:
- Move FormData + createReadStream inside the retry loop (fresh stream per attempt)
- Use (whisperJob.segments ?? []) in webhook handler
- Add Array.isArray guard at top of deduplicateSegments() as belt-and-suspenders

Tests:
- New: verifies createReadStream called once per attempt (3 attempts = 3 streams)
- New: webhook handles segments:undefined without throwing
- New: webhook handles segments:null without throwing
- 150/150 passing

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

src/tests/webhook.test.ts

@@ -289,6 +289,34 @@ describe('POST /api/webhook/[jobId] — empty segments', () => {
 	});
 });
 
+// ── Undefined / missing segments (model returned no segments field) ───────────
+
+describe('POST /api/webhook/[jobId] — undefined segments', () => {
+	it('completes the job as done when segments field is absent from whisper payload', async () => {
+		mockGetJob.mockReturnValue(makeJob('job-noseg'));
+		// Simulate whisper returning a result without a segments field
+		const payload = { ...makeWhisperJob(), segments: undefined as unknown as never[] };
+
+		const res = await POST(makeEvent('job-noseg', payload) as any);
+		expect(res.status).toBe(200);
+		expect(await res.json()).toEqual({ ok: true });
+		expect(mockUpdateJob).toHaveBeenCalledWith(
+			expect.objectContaining({ status: 'done', id: 'job-noseg' })
+		);
+	});
+
+	it('does not throw "cannot read properties of undefined" when segments is null', async () => {
+		mockGetJob.mockReturnValue(makeJob('job-nullseg'));
+		const payload = { ...makeWhisperJob(), segments: null as unknown as never[] };
+
+		// Must NOT throw — previously crashed with "Cannot read properties of undefined (reading 'map')"
+		await expect(POST(makeEvent('job-nullseg', payload) as any)).resolves.toBeDefined();
+		expect(mockUpdateJob).toHaveBeenCalledWith(
+			expect.objectContaining({ status: 'done' })
+		);
+	});
+});
+
 // ── Internal error handling ───────────────────────────────────────────────────
 
 describe('POST /api/webhook/[jobId] — internal errors', () => {