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/features/admin/integrations/facade/admin-integrations.facade.ts

@@ -0,0 +1,28 @@
+import { Injectable, inject, signal } from '@angular/core';
+import { take } from 'rxjs/operators';
+import { Connector } from '../../../../core/integrations/models/connector.model';
+import { CONNECTOR_GATEWAY } from '../../../../core/integrations/services/connector-gateway.token';
+
+@Injectable({ providedIn: 'root' })
+export class AdminIntegrationsFacade {
+  private readonly gateway = inject(CONNECTOR_GATEWAY);
+
+  readonly connectors = signal<Connector[]>([]);
+  readonly loading = signal(false);
+
+  load(): void {
+    this.loading.set(true);
+    this.gateway.loadConnectors().pipe(take(1)).subscribe(items => {
+      this.connectors.set(items);
+      this.loading.set(false);
+    });
+  }
+
+  pause(id: string): void {
+    this.gateway.pause(id).pipe(take(1)).subscribe(() => this.load());
+  }
+
+  resume(id: string): void {
+    this.gateway.resume(id).pipe(take(1)).subscribe(() => this.load());
+  }
+}