feat: add Agrarmonitor correspondent sync

- Extract getOrCreateCorrespondent helper to deduplicate logic
- Add syncCorrespondentIds to match Paperless correspondents to
  Agrarmonitor IDs via Lieferantennummer and persist in CorrespondentSetting
- New POST /api/agrarmonitor/sync-correspondents endpoint
- "Agrarmonitor-Abgleich" button in Correspondents settings tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-backend/src/agrarmonitor/agrarmonitor.module.ts

@@ -6,10 +6,11 @@ import { AgrarmonitorController } from './agrarmonitor.controller';
 import { PaperlessModule } from '../paperless/paperless.module';
 import { Setting } from '../database/entities/setting.entity';
 import { Client } from '../database/entities/client.entity';
+import { CorrespondentSetting } from '../database/entities/correspondent-setting.entity';
 
 @Module({
   imports: [
-    TypeOrmModule.forFeature([Setting, Client]),
+    TypeOrmModule.forFeature([Setting, Client, CorrespondentSetting]),
     PaperlessModule,
   ],
   providers: [AgrarmonitorService, AgrarmonitorPollingService],