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/prisma/migrations/20260822204038_notifications/migration.sql

@@ -0,0 +1,25 @@
+-- CreateEnum
+CREATE TYPE "NotificationPriority" AS ENUM ('INFO', 'HINWEIS', 'WARNUNG', 'KRITISCH');
+
+-- CreateTable
+CREATE TABLE "Notification" (
+    "id" TEXT NOT NULL,
+    "userId" TEXT,
+    "priority" "NotificationPriority" NOT NULL,
+    "message" TEXT NOT NULL,
+    "relatedType" TEXT,
+    "relatedId" TEXT,
+    "read" BOOLEAN NOT NULL DEFAULT false,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "Notification_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "Notification_userId_read_idx" ON "Notification"("userId", "read");
+
+-- CreateIndex
+CREATE INDEX "Notification_priority_idx" ON "Notification"("priority");
+
+-- AddForeignKey
+ALTER TABLE "Notification" ADD CONSTRAINT "Notification_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;