feat: expand backup to all data and add automatic scheduled backups

Backup export/import now covers Role, User, RolePermission,
Notification, AuditLog and Document (with actual file content), not
just business data - matching "wirklich alle Daten". Role/User use
natural keys (role.key, resource+action) so a bundle restores cleanly
into a different database.

Adds BackupSchedulerService (@nestjs/schedule) for cron-based automatic
backups to a local path (BACKUP_STORAGE_PATH), with retention pruning
and a file-mtime guard against duplicate runs across replicas. New
GET /backup/schedule and POST /backup/schedule/run-now endpoints, plus
a status card and manual trigger in the frontend Backup settings page.

ApiCore-Backend/src/app.module.ts

@@ -2,6 +2,7 @@ import { Module } from '@nestjs/common';
 import { ConfigModule } from '@nestjs/config';
 import { APP_GUARD } from '@nestjs/core';
 import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
+import { ScheduleModule } from '@nestjs/schedule';
 import { PrismaModule } from './prisma/prisma.module';
 import { HealthModule } from './health/health.module';
 import { AuditModule } from './audit/audit.module';
@@ -49,6 +50,7 @@ import { PermissionsModule } from './auth/permissions/permissions.module';
 @Module({
   imports: [
     ConfigModule.forRoot({ isGlobal: true }),
+    ScheduleModule.forRoot(),
     ThrottlerModule.forRoot([{ name: 'default', ttl: 60000, limit: 100 }]),
     PrismaModule,
     PermissionsModule,