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/__tests__/stack-update-post-recheck.test.ts

@@ -37,6 +37,7 @@ vi.mock('../services/ImageUpdateService', async () => {
   return {
     ...actual,
     ImageUpdateService: {
+      isChecksEnabled: () => true,
       getInstance: () => ({ recheckStack: mockRecheckStack }),
     },
   };