feat: add cron scheduling mode for image update checks (#1460)

* feat: add cron scheduling mode for image update checks

Adds a cron scheduling mode alongside the existing fixed-interval
dropdown in Settings > Automation > Image update checks. Users can
now set a 5-field cron expression (e.g. "0 3 * * 1") for precise
time-of-day scheduling of registry polls.

- Backend: ImageUpdateService gains mode/cronExpression fields and
  cron-based nextDelayMs() using the existing cron-parser dependency.
  PUT /api/image-updates/interval extended with transactional writes
  and server-authoritative cron validation matching the Scheduled
  Operations contract. Nicknames like @daily are supported.
- Frontend: UpdatesSection gains a SegmentedControl toggle and cron
  text input with cronstrue-powered live description. The frontend
  does advisory validation only; backend 400s are surfaced inline.
  SettingsPrimaryButton used for explicit "Save schedule" action.
- No cron jitter (the user chose a specific time). Interval mode
  keeps existing ±10% jitter.
- Tests: 15 new backend tests covering valid cron, invalid cron,
  6-field rejection, nickname support, backward compat, runtime
  fallback, and transactional writes.
- Docs: auto-update-policies.mdx, alerts-notifications.mdx, and
  openapi.yaml updated with new scheduling mode.

* fix: add mode and cronExpression to UpdatesSection test fixtures

The existing tests failed because the mock status object was missing
the new required fields (mode, cronExpression) added with cron
scheduling support. Without them, status.mode was undefined, causing
uiMode to never match 'interval' and the Select combobox to not render.

* fix: prevent SegmentedControl from stretching full-width in SettingsField

The flex-col container defaults items to align-self: stretch, making the
Interval/Cron toggle bar span the full card width. Add self-start so it
sizes to its content.

backend/src/__tests__/image-updates-routes.test.ts

@@ -122,6 +122,94 @@ describe('PUT /api/image-updates/interval', () => {
     const statusRes = await request(app).get('/api/image-updates/status').set('Cookie', adminCookie);
     expect(statusRes.body.intervalMinutes).toBe(30);
   });
+
+  // ── Cron mode ──────────────────────────────────────────────────────────
+
+  it('persists a valid cron expression and returns the enriched status', async () => {
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '0 3 * * 1' });
+    expect(res.status).toBe(200);
+    expect(res.body.mode).toBe('cron');
+    expect(res.body.cronExpression).toBe('0 3 * * 1');
+    const settings = DatabaseService.getInstance().getGlobalSettings();
+    expect(settings.image_update_check_mode).toBe('cron');
+    expect(settings.image_update_check_cron).toBe('0 3 * * 1');
+  });
+
+  it('accepts a cron nickname like @daily', async () => {
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '@daily' });
+    expect(res.status).toBe(200);
+    expect(res.body.mode).toBe('cron');
+    expect(res.body.cronExpression).toBe('@daily');
+  });
+
+  it('rejects a 6-field cron expression', async () => {
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '0 0 3 * * 1' });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/5 fields/);
+  });
+
+  it('rejects a blank cron expression when mode is cron', async () => {
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '   ' });
+    expect(res.status).toBe(400);
+  });
+
+  it('rejects an invalid cron expression (backend-authoritative)', async () => {
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '0 0 31 2 *' });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/Invalid cron/);
+  });
+
+  it('rejects cron mode without a cron field', async () => {
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron' });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/Cron expression/);
+  });
+
+  it('clears cron when switching back to interval mode', async () => {
+    // First set cron mode.
+    await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '0 3 * * 1' });
+    // Then switch to interval.
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 60, mode: 'interval' });
+    expect(res.status).toBe(200);
+    expect(res.body.mode).toBe('interval');
+    expect(res.body.cronExpression).toBeNull();
+    const settings = DatabaseService.getInstance().getGlobalSettings();
+    expect(settings.image_update_check_mode).toBe('interval');
+    expect(settings.image_update_check_cron).toBe('');
+  });
+
+  it('old-client { minutes } only does not change mode (backward compat)', async () => {
+    // First set cron mode.
+    await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 120, mode: 'cron', cron: '0 3 * * 1' });
+    // Then send old-client payload.
+    const res = await request(app).put('/api/image-updates/interval')
+      .set('Cookie', adminCookie)
+      .send({ minutes: 30 });
+    expect(res.status).toBe(200);
+    // Mode and cron are unchanged.
+    expect(res.body.mode).toBe('cron');
+    expect(res.body.cronExpression).toBe('0 3 * * 1');
+    // Interval was updated (the fallback value).
+    expect(res.body.intervalMinutes).toBe(30);
+  });
 });
 
 describe('GET /api/image-updates/fleet', () => {