Backend: GET /api/system/version-check fetches frontend/package.json from
the main branch on GitHub, caches result in app_settings for 24 hours,
and compares semver against the running version. Fails silently (no banner)
for air-gapped or firewalled deployments.
Frontend: dismissible blue update banner in SecurityBanners (rendered
between TopBar and main content) — only visible when a newer version is
detected. Shows target version and docker compose update command.