feat(backend): Benachrichtigungssystem (Modul 26)

NotificationsModule: Notification-Modell mit Prioritaeten INFO/
HINWEIS/WARNUNG/KRITISCH, GET /notifications (eigene + Broadcast),
POST /notifications/:id/read. record() als interne API fuer
kuenftige Automatisierungsregeln (Modul 30) vorgesehen.

ApiCore-Backend/src/notifications/notifications.module.ts

@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { NotificationsController } from './notifications.controller';
+import { NotificationsService } from './notifications.service';
+
+@Module({
+  controllers: [NotificationsController],
+  providers: [NotificationsService],
+  exports: [NotificationsService],
+})
+export class NotificationsModule {}