feat(admin): monitoring center

Sprint 26.

New features/admin/monitoring/ module + net-new /:lang/backoffice/monitoring
route + Dashboard Quick Action.

- Health section reuses AdminDashboardFacade.healthChecks directly (real
  data, unchanged since Sprint 19) instead of duplicating the logic
- unified AdminMonitoringEvent feed covering audit/security/login/
  failed-login/api/error/warning, category filter + search, 40 seeded
  synthetic entries (no logging backend exists anywhere in this system)
- mock queue depth/status cards, mock webhook delivery log
- intentionally kept separate from Sprint 24's per-transaction audit and
  Sprint 25's per-user audit - different scopes, no consolidation attempted

Also fixed a real type error: AdminDashboardQuickActionId's union was
missing 'users' and 'monitoring' (added when wiring those Quick Actions),
caught by ng build's template type-checking even though plain tsc --noEmit
passed - a reminder that ng build is the authoritative check here.

docs/ADMIN.md + docs/BACKEND.md (new item 15) updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/features/admin/dashboard/facade/admin-dashboard.facade.ts

@@ -19,6 +19,7 @@ const QUICK_ACTIONS: AdminDashboardQuickAction[] = [
   { id: 'orders', labelKey: 'dashboard.actionOrders', route: ['backoffice', 'orders'] },
   { id: 'media-library', labelKey: 'dashboard.actionMediaLibrary', route: ['backoffice', 'media'] },
   { id: 'users', labelKey: 'dashboard.actionUsers', route: ['backoffice', 'users'] },
+  { id: 'monitoring', labelKey: 'dashboard.actionMonitoring', route: ['backoffice', 'monitoring'] },
   { id: 'preview-marketplace', labelKey: 'dashboard.actionPreviewMarketplace', route: [''] },
 ];