feat: Phase 4 frontend - Integrations backoffice section (mock-gateway backed)

New core/integrations (Connector/DeadLetterEntry models + gateway/token)
and features/admin/integrations (facade + page: connector table with
status/lag/errors/backlog/unmatched, pause/resume). Seeded empty per
Sprint 0.1's "no fixed partner list" decision - the section is ready to
populate the moment the first real connector is onboarded against
docs/backend/PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md §7. New /backoffice/
integrations route + nav entry, nav i18n key in all 3 languages.

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

src/app/app.routes.ts

@@ -200,6 +200,15 @@ const coreRoutes: Routes = [
           breadcrumb: [{ labelKey: 'adminShell.nav.notifications' }]
         }
       },
+      {
+        path: 'integrations',
+        loadComponent: () => import('./features/admin/integrations/pages/admin-integrations-page.component').then(m => m.AdminIntegrationsPageComponent),
+        data: {
+          titleKey: 'adminShell.nav.integrations',
+          descriptionKey: 'adminShell.nav.integrations',
+          breadcrumb: [{ labelKey: 'adminShell.nav.integrations' }]
+        }
+      },
       {
         path: 'moderation',
         loadComponent: () => import('./features/admin/moderation/pages/admin-reviews-list-page.component').then(m => m.AdminReviewsListPageComponent),