feat(schedules): next-24h timeline + merge auto-update into schedules (#681)

* feat(backend): add stack update-preview endpoint for readiness board

Adds GET /api/stacks/:stackName/update-preview that returns per-image
semver diff, bump classification, and a stack-level summary powering
the Auto-Update readiness board.

- New UpdatePreviewService parses compose images, inspects local
  digests, fetches remote digests and tag lists, and finds the
  highest compatible semver tag.
- Major bumps are flagged blocked until human review; unknown bumps
  rank below real semver so they cannot mask a major.
- Rollback target is reconstructed through parseImageRef to preserve
  registry ports and drop the Docker Hub library/ prefix.
- Registry helpers (httpGet, auth token, digest, tag list, ref parse)
  are extracted into registry-api.ts and shared with ImageUpdateService.
- 28 Vitest cases cover parse, selection, bump math, digest rebuilds,
  blocked policy, and rollback target construction.

* feat(schedules): next-24h timeline, merge auto-update crud, add readiness board

Replace the flat task table with a Timeline view as the default, showing the
next 24 hours of scheduled work across four lanes (Restart, Update, Scan,
Prune) with a live now rail and per-firing pills. The All tasks tab preserves
the existing CRUD surface.

Merge Auto-update Stack into Schedules as a first-class action and replace the
standalone Auto-Update Policies view with a per-stack Readiness board that
surfaces version diffs, risk tags, changelog previews, and rollback targets
sourced from the stack update-preview endpoint.

frontend/src/components/EditorLayout.tsx

@@ -54,7 +54,7 @@ import { GlobalObservabilityView } from './GlobalObservabilityView';
 import { FleetView } from './FleetView';
 import { AuditLogView } from './AuditLogView';
 import ScheduledOperationsView from './ScheduledOperationsView';
-import AutoUpdatePoliciesView from './AutoUpdatePoliciesView';
+import AutoUpdateReadinessView from './AutoUpdateReadinessView';
 import { SecurityHistoryView } from './SecurityHistoryView';
 import { SENCHO_NAVIGATE_EVENT } from './NodeManager';
 import type { SenchoNavigateDetail } from './NodeManager';
@@ -2779,8 +2779,8 @@ export default function EditorLayout() {
               <AuditLogView />
             </CapabilityGate>
           ) : activeView === 'auto-updates' ? (
-            <CapabilityGate capability="auto-updates" featureName="Auto-Update Policies">
-              <AutoUpdatePoliciesView filterNodeId={filterNodeId} onClearFilter={() => setFilterNodeId(null)} />
+            <CapabilityGate capability="auto-updates" featureName="Auto-Update Readiness">
+              <AutoUpdateReadinessView />
             </CapabilityGate>
           ) : activeView === 'scheduled-ops' ? (
             <CapabilityGate capability="scheduled-ops" featureName="Scheduled Operations">