feat: high-availability page (/ha)

Platform-level HA surface: manager enable/disable toggle, status cards
(nodes, health checks, active alerts), manual failover with
confirmation, per-service failover policy CRUD with project/service
picker, active alert resolution, and health check result list.

Verified live: status, enable/disable, policy set/list/get/disable,
manual failover, alerts and health results endpoints. HA state is
in-memory — noted in roadmap alongside autoscaling persistence debt.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/src/app/layout/PlatformShell.tsx

@@ -15,6 +15,7 @@ import {
   RefreshCw,
   Search,
   Settings,
+  ShieldCheck,
   Sun,
   UploadCloud,
   Users,
@@ -34,6 +35,7 @@ const navItems = [
   { label: 'Projects', href: '/projects', icon: FolderKanban },
   { label: 'Templates', href: '/templates', icon: LayoutTemplate },
   { label: 'Databases', href: '/databases', icon: Database },
+  { label: 'HA', href: '/ha', icon: ShieldCheck },
   { label: 'Usage', href: '/usage', icon: ChartBar },
   { label: 'People', href: '/people', icon: Users },
   { label: 'Settings', href: '/settings', icon: Settings },