Add Databases management page and roadmap coverage audit

/databases page (new nav item): expandable managed-DB cards with
connection URL + copy, start/stop/restart actions, metrics summary, and
backup list with create/restore. Verified e2e against a real postgres:16
container — provision, psql connect, stop/start, seed, 6.4MB backup,
drop, restore with data recovered.

ROADMAP: marked databases/cron/git-connect/agents done with evidence,
corrected stale inventory rows (frontend now consumes 52 API ops),
added uncovered items — GitHub App install surfacing, database→service
binding, backup export + scheduled backups, service exec console,
OAuth e2e w/ real creds, remaining sqlc port, demo-data parity.

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

@@ -7,6 +7,7 @@ import {
   ChartBar,
   CheckCircle2,
   Container,
+  Database,
   FolderKanban,
   LayoutTemplate,
   LogOut,
@@ -31,6 +32,7 @@ import { useToast } from '@/shared/components';
 const navItems = [
   { label: 'Projects', href: '/projects', icon: FolderKanban },
   { label: 'Templates', href: '/templates', icon: LayoutTemplate },
+  { label: 'Databases', href: '/databases', icon: Database },
   { label: 'Usage', href: '/usage', icon: ChartBar },
   { label: 'People', href: '/people', icon: Users },
   { label: 'Settings', href: '/settings', icon: Settings },