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.

backend/src/services/SchedulerService.ts

@@ -1038,6 +1038,10 @@ export class SchedulerService {
         imageUpdateService: ImageUpdateService,
         isWildcard = false
     ): Promise<string> {
+        if (!ImageUpdateService.isChecksEnabled()) {
+            console.log(`[SchedulerService] Stack "${stackName}": image update detection is disabled; skipped.`);
+            return `Stack "${stackName}": image update detection is disabled; skipped.`;
+        }
         const containers = await docker.getContainersByStack(stackName);
         if (!containers || containers.length === 0) {
             if (!isWildcard) {