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.

backend/src/services/SchedulerService.ts

@@ -140,6 +140,21 @@ export class SchedulerService {
         return expr.next().toDate().getTime();
     }
 
+    public calculateRunsWithin(cronExpression: string, fromMs: number, toMs: number, limit = 16): number[] {
+        try {
+            const expr = CronExpressionParser.parse(cronExpression, { currentDate: new Date(fromMs) });
+            const runs: number[] = [];
+            while (runs.length < limit) {
+                const next = expr.next().toDate().getTime();
+                if (next > toMs) break;
+                runs.push(next);
+            }
+            return runs;
+        } catch {
+            return [];
+        }
+    }
+
     /**
      * Fire a notification without awaiting completion, catching any promise
      * rejection so the scheduler never crashes on a failed dispatch.