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-update-service.test.ts

@@ -988,3 +988,111 @@ services:
     expect(checkedImages).not.toContain('someapp:v2');
   });
 });
+
+describe('ImageUpdateService cron scheduling', () => {
+  beforeEach(() => {
+    (ImageUpdateService as any).instance = undefined;
+    mockGetGlobalSettings.mockReturnValue({ developer_mode: '0' });
+  });
+
+  it('getStatus returns mode and cronExpression fields', () => {
+    const service = ImageUpdateService.getInstance();
+    // Before start/configureFromSettings, defaults apply.
+    mockGetGlobalSettings.mockReturnValue({ developer_mode: '0' });
+    service.configureFromSettings();
+    const status = service.getStatus();
+    expect(status.mode).toBe('interval');
+    expect(status.cronExpression).toBeNull();
+  });
+
+  it('configureFromSettings sets cron mode with valid expression', () => {
+    mockGetGlobalSettings.mockReturnValue({
+      developer_mode: '0',
+      image_update_check_mode: 'cron',
+      image_update_check_cron: '0 3 * * 1',
+      image_update_check_interval_minutes: '120',
+    });
+    const service = ImageUpdateService.getInstance();
+    service.configureFromSettings();
+    const status = service.getStatus();
+    expect(status.mode).toBe('cron');
+    expect(status.cronExpression).toBe('0 3 * * 1');
+  });
+
+  it('configureFromSettings falls back to interval on invalid cron', () => {
+    mockGetGlobalSettings.mockReturnValue({
+      developer_mode: '0',
+      image_update_check_mode: 'cron',
+      image_update_check_cron: 'not a cron expression',
+      image_update_check_interval_minutes: '120',
+    });
+    const service = ImageUpdateService.getInstance();
+    service.configureFromSettings();
+    const status = service.getStatus();
+    expect(status.mode).toBe('interval');
+    expect(status.cronExpression).toBeNull();
+  });
+
+  it('configureFromSettings falls back to interval when cron mode has empty expression', () => {
+    mockGetGlobalSettings.mockReturnValue({
+      developer_mode: '0',
+      image_update_check_mode: 'cron',
+      image_update_check_cron: '',
+      image_update_check_interval_minutes: '120',
+    });
+    const service = ImageUpdateService.getInstance();
+    service.configureFromSettings();
+    const status = service.getStatus();
+    expect(status.mode).toBe('interval');
+    expect(status.cronExpression).toBeNull();
+  });
+
+  it('configureFromSettings accepts cron nicknames like @daily', () => {
+    mockGetGlobalSettings.mockReturnValue({
+      developer_mode: '0',
+      image_update_check_mode: 'cron',
+      image_update_check_cron: '@daily',
+      image_update_check_interval_minutes: '120',
+    });
+    const service = ImageUpdateService.getInstance();
+    service.configureFromSettings();
+    const status = service.getStatus();
+    expect(status.mode).toBe('cron');
+    expect(status.cronExpression).toBe('@daily');
+  });
+
+  it('nextDelayMs computes a positive delay for a valid cron expression', () => {
+    mockGetGlobalSettings.mockReturnValue({
+      developer_mode: '0',
+      image_update_check_mode: 'cron',
+      image_update_check_cron: '0 3 * * 1',
+      image_update_check_interval_minutes: '120',
+    });
+    const service = ImageUpdateService.getInstance();
+    service.configureFromSettings();
+    // nextDelayMs is private; access it to verify it does not throw and returns
+    // a positive number (next Monday at 03:00 is in the future).
+    const delay = (service as any).nextDelayMs();
+    expect(typeof delay).toBe('number');
+    expect(delay).toBeGreaterThan(0);
+  });
+
+  it('nextDelayMs falls back to interval on runtime parse failure', () => {
+    // Set up cron mode, then corrupt the expression at runtime before nextDelayMs.
+    mockGetGlobalSettings.mockReturnValue({
+      developer_mode: '0',
+      image_update_check_mode: 'cron',
+      image_update_check_cron: '0 3 * * 1',
+      image_update_check_interval_minutes: '120',
+    });
+    const service = ImageUpdateService.getInstance();
+    service.configureFromSettings();
+    // Corrupt the expression directly on the private field.
+    (service as any).cronExpression = '0 0 31 2 *'; // Feb 31 — invalid
+    const delay = (service as any).nextDelayMs();
+    // Should fall back to interval mode after the parse error.
+    expect(service.getStatus().mode).toBe('interval');
+    expect(typeof delay).toBe('number');
+    expect(delay).toBeGreaterThan(0);
+  });
+});