feat(security): severity-aware scheduled scan notifications (#654)

Enrich scheduled vulnerability scan completion notifications with
per-severity CVE counts so recipients can triage from the message body
alone. Expose the scan action in the schedule creation UI, require an
explicit node_id, and harden fire-and-forget alert dispatches so a
failing webhook cannot crash the scheduler.

Notification body now reports scanned/skipped/failed counts plus
critical/high/medium totals aggregated across fresh and cached scans,
reflecting the current node posture rather than only what was newly
scanned on this run.

frontend/src/types/scheduling.ts

@@ -4,7 +4,7 @@ export interface ScheduledTask {
   target_type: 'stack' | 'fleet' | 'system';
   target_id: string | null;
   node_id: number | null;
-  action: 'restart' | 'snapshot' | 'prune' | 'update';
+  action: 'restart' | 'snapshot' | 'prune' | 'update' | 'scan';
   cron_expression: string;
   enabled: number;
   created_by: string;