feat: add node-scoped opt-out for image update detection (#1715)

* feat: add node-scoped opt-out for image update detection

Operators who use an external update authority can disable Sencho registry
polling per node without losing explicit stack Update, pull, or redeploy.

* test: fix mocks and lint for image-update checks opt-out

Scheduler tests need isChecksEnabled on the ImageUpdateService mock, and the UpdatesSection older-node fixture must not leave an unused binding.

* fix: gate update-preview and recheck when detection is off

Anatomy was still calling stack update-preview (and contacting registries)
while checks were disabled. Short-circuit those routes and skip recheckStack
writes so disabled nodes stay quiet until detection is re-enabled.

frontend/src/types/imageUpdates.ts

@@ -24,6 +24,11 @@ export interface ImageUpdateStatus {
     cronExpression: string | null;
     /** Whether sidebar update-status indicators are enabled. Optional for older-node compatibility. */
     sidebarIndicators?: boolean;
+    /**
+     * Whether background image-update detection is armed. Optional for older
+     * remotes; absence means the node does not support the opt-out yet.
+     */
+    enabled?: boolean;
 }
 
 /**