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.service.ts

@@ -0,0 +1,51 @@
+import { Injectable, NotFoundException } from '@nestjs/common';
+import { PrismaService } from '../prisma/prisma.service';
+import { Notification, NotificationPriority } from '../../generated/prisma';
+
+export interface CreateNotificationInput {
+  userId?: string;
+  priority: NotificationPriority;
+  message: string;
+  relatedType?: string;
+  relatedId?: string;
+}
+
+export interface NotificationFilter {
+  userId?: string;
+  unreadOnly?: boolean;
+}
+
+@Injectable()
+export class NotificationsService {
+  constructor(private readonly prisma: PrismaService) {}
+
+  async record(input: CreateNotificationInput): Promise<Notification> {
+    return this.prisma.notification.create({ data: { ...input } });
+  }
+
+  async findAll(filter: NotificationFilter = {}): Promise<Notification[]> {
+    return this.prisma.notification.findMany({
+      where: {
+        OR: filter.userId
+          ? [{ userId: filter.userId }, { userId: null }]
+          : undefined,
+        read: filter.unreadOnly ? false : undefined,
+      },
+      orderBy: { createdAt: 'desc' },
+      take: 100,
+    });
+  }
+
+  async markRead(id: string): Promise<Notification> {
+    const notification = await this.prisma.notification.findUnique({
+      where: { id },
+    });
+    if (!notification) {
+      throw new NotFoundException('Notification not found');
+    }
+    return this.prisma.notification.update({
+      where: { id },
+      data: { read: true },
+    });
+  }
+}