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.

docs/features/alerts-notifications.mdx

@@ -384,7 +384,7 @@ The **Host Alerts** panel also carries the **Host thresholds** rows (CPU limit,
 | Crash, OOM, and healthcheck events | Real-time over the Docker event stream |
 | Host CPU / RAM / disk threshold checks | 30 seconds |
 | Per-stack alert rule evaluation | 30 seconds |
-| Image update poll | 6 hours, with a 2-minute startup delay and a 2-minute cooldown on manual refresh |
+| Image update poll | Configurable (default every 2 hours), with a 2-minute startup delay and a 2-minute cooldown on manual refresh |
 | Sencho version check | 6 hours |
 | Notification fanout to channels | Single shot per dispatch, 10-second timeout, no retries |
 | Bell live updates | Pushed live over the notifications WebSocket per node |

docs/features/auto-update-policies.mdx

@@ -33,7 +33,7 @@ When nothing is pending, the board renders a single Shield-icon panel with the h
 
 ## Detection cadence
 
-Sencho polls your registries on a set interval to detect available image updates and raise notifications. This detection cadence is configurable under **Settings > Automation > Image update checks**: choose anything from every 15 minutes to once a day. The default is every 2 hours, and changing it takes effect immediately, with no restart.
+Sencho polls your registries on a configurable schedule to detect available image updates and raise notifications. This detection cadence is configurable under **Settings > Automation > Image update checks**: choose a fixed interval (every 15 minutes to once a day) or set a cron expression for precise scheduling (e.g. "every Monday at 03:00"). The default is every 2 hours on an interval schedule, and changing it takes effect immediately, with no restart.
 
 Detection is separate from applying updates: