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.

docs/operations/trivy-setup.mdx

@@ -36,6 +36,7 @@ Behind the scenes:
 - The Trivy binary is downloaded into Sencho's existing data volume at `/app/data/bin/trivy`. No host filesystem changes.
 - The vulnerability database cache defaults to `/app/data/trivy-cache` so it persists across container restarts.
 - Downloads are verified against the official Trivy checksum file before the binary is put in place.
+- By default the managed install pins to a known-good Trivy version, so a fresh install is reproducible rather than tracking whatever "latest" happens to be. Turn on **Auto-update Trivy** to track the newest release instead; an explicit **Update** always pulls the latest regardless of the toggle.
 - The installed version survives Sencho image upgrades because it lives on the mounted data volume.
 
 ### Updating the managed install