feat(images): Trivy-powered vulnerability scanning (#635)

* feat(images): Trivy-powered vulnerability scanning

Scan container images for known CVEs via Trivy. On-demand scanning and
severity badges are available on every tier; scheduled scans, scan
policies, SBOM generation, and scan history are gated to Skipper+.

- New TrivyService (binary detection, per-image scan, SBOM, digest cache)
- Three new tables: vulnerability_scans, vulnerability_details, scan_policies
- 12 routes under /api/security (scan, results, summaries, SBOM, policies, compare)
- Post-deploy async scans wired into all five deploy paths, with a
  per-deploy opt-out toggle in the App Store deploy sheet
- "scan" action type added to SchedulerService for fleet-wide recurring scans
- Frontend: severity badges in Resources Hub with animated cursor detail,
  scan results drawer with vulnerability table and filters, and a new
  Security section in Settings for scan policy CRUD
- Policy threshold violations dispatch a warning or critical alert based on
  the policy's block_on_deploy flag; deploys themselves are never blocked

* fix(security): compute scan age in useEffect to satisfy react-hooks/purity

backend/src/services/CapabilityRegistry.ts

@@ -33,6 +33,7 @@ export const CAPABILITIES = [
   'users',
   'registries',
   'self-update',
+  'vulnerability-scanning',
 ] as const;
 
 export type Capability = (typeof CAPABILITIES)[number];