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/app.routes.ts

@@ -112,6 +112,10 @@ const coreRoutes: Routes = [
         path: 'users',
         loadComponent: () => import('./features/admin/users/pages/admin-users-page.component').then(m => m.AdminUsersPageComponent)
       },
+      {
+        path: 'monitoring',
+        loadComponent: () => import('./features/admin/monitoring/pages/admin-monitoring-page.component').then(m => m.AdminMonitoringPageComponent)
+      },
       { path: '**', redirectTo: 'dashboard' }
     ]
   },