feat: make image-update check cadence configurable and visible (#1377)

* feat: make image-update check cadence configurable and visible

The background image-update scanner polled registries on a hardcoded
6-hour interval, with no way to see when it last ran or when the next
run was due. Operators testing updates read this as auto-update being
unreliable: a manual update checks the registry immediately and applies,
so the slow background scan rarely raised the "update available"
notification before the stack was already current.

Backend:
- ImageUpdateService reads image_update_check_interval_minutes (15-1440,
  default 120) and drives a single generation-guarded self-rescheduling
  timer with 10% per-run jitter so fleet nodes do not poll in lockstep.
  restartPolling() applies a new interval live, with no restart, and
  cannot leave a duplicate timer when a save lands mid-scan.
- GET /api/image-updates/status now returns checking, intervalMinutes,
  lastCheckedAt, nextCheckAt, and the manual-cooldown fields. New
  admin-only PUT /api/image-updates/interval persists the setting and
  reschedules.

Frontend:
- New Settings > Automation > Image update checks section to choose the
  interval (read-only for non-admins; admin enforced on the backend).
- The Auto-Update readiness view shows last-checked, next-check, and a
  ticking manual-recheck cooldown, and the copy distinguishes registry
  detection from scheduled auto-update execution.

Adds backend unit and route tests and frontend component tests, and
updates the auto-update documentation.

* fix: drop stale image-update status response in the readiness strip

loadCadence() ran on mount and again after a Recheck with no request
token, so a slow initial /image-updates/status response could resolve
after the recheck-triggered one and overwrite the fresh cooldown with
stale data, or set state after the view unmounted. Guard setCadence with
a monotonic token mirroring loadReadiness, and bump it on unmount. Adds a
regression test for the out-of-order resolution.

frontend/src/types/imageUpdates.ts

@@ -0,0 +1,21 @@
+/**
+ * Status of a node's image-update scanner, as returned by
+ * `GET /api/image-updates/status`. Shared by the Settings cadence section and
+ * the Auto-Update readiness strip so the wire shape lives in one place.
+ *
+ * Units differ by field: `intervalMinutes` and `manualCooldownMinutes` are
+ * minutes; `manualCooldownRemainingMs` is milliseconds (the UI needs ms to tick
+ * a 1-second countdown). `manualCooldownMinutes` is a fixed ceiling (the
+ * cooldown window), while `manualCooldownRemainingMs` is the live remaining
+ * time (0 when a manual refresh is allowed). `lastCheckedAt` / `nextCheckAt`
+ * are epoch-ms or null ("never checked" / "not scheduled"); `nextCheckAt` is
+ * ignored while `checking` is true.
+ */
+export interface ImageUpdateStatus {
+    checking: boolean;
+    intervalMinutes: number;
+    lastCheckedAt: number | null;
+    nextCheckAt: number | null;
+    manualCooldownMinutes: number;
+    manualCooldownRemainingMs: number;
+}