feat: pre-deploy scan visibility and pinned scanner version (#1378)

* feat: pre-deploy scan visibility and pinned scanner version

Pin managed Trivy installs and add an opt-in pre-deploy scan advisory so a
manual deploy can surface each image's latest scan before it runs.

- Managed Trivy now installs a pinned, known-good version by default for
  reproducible installs. Auto-update still tracks the latest release, and an
  explicit update always pulls the latest.
- Add an opt-in pre-deploy scan advisory: when enabled, deploying a stack from
  the editor first shows each image's latest cached scan severity for review.
  It is visibility only and never blocks; deploy enforcement is unchanged.
- Backend: pre_deploy_scan_advisory setting, PUT
  /security/pre-deploy-scan-advisory, a cache-only GET
  /security/stacks/:name/pre-deploy-summary, and a node-scoped
  getLatestVulnScanByDigestForNode lookup.
- Frontend: advisory toggle on the Security page scanner setup, and a
  PreDeployScanDialog wired into the editor deploy flow that fails open when the
  summary is unavailable.
- Docs: scanner configuration, version pinning, and the advisory.

* fix: harden pre-deploy advisory guard, toggle visibility, and installer busy state

Addresses review findings on the pre-deploy advisory.

- Block a second editor deploy during the async advisory window with a
  synchronous pending ref, cleared on cancel and in the deploy's finally, so a
  double-click can no longer start two deploys.
- Keep the pre-deploy advisory toggle visible to admins whenever the setting is
  on, so it can still be turned off after the scanner becomes unavailable.
- Resolve the managed Trivy version inside the install lock so the busy state and
  serialization cover the latest-version fetch and the managed-install check.

frontend/src/hooks/useTrivyStatus.ts

@@ -8,6 +8,7 @@ const INITIAL_STATUS: TrivyStatus = {
   source: 'none',
   autoUpdate: false,
   honorSuppressionsOnDeploy: false,
+  preDeployScanAdvisory: false,
   busy: false,
 };
 
@@ -33,6 +34,7 @@ export function useTrivyStatus(): UseTrivyStatusResult {
         source: d.source === 'managed' || d.source === 'host' ? d.source : 'none',
         autoUpdate: !!d.autoUpdate,
         honorSuppressionsOnDeploy: !!d.honorSuppressionsOnDeploy,
+        preDeployScanAdvisory: !!d.preDeployScanAdvisory,
         busy: !!d.busy,
       });
     } catch (err) {