fix: two polling bugs — correspondent 400 and Agrarmonitor 403

- 400 on Korrespondenten-Sync: getCorrespondentByName was called with
  searchName "(12345)" but checked exact match against full displayName
  "Firma (12345)". Always returned null → duplicate addCorrespondent on
  every run → Paperless 400. Fix: search by displayName directly.

- 403 on Livesearch: cached Agrarmonitor session expired. Fix: detect
  401/403 from connector, call clearClient() to invalidate cache, break
  out of the polling loop so next cron run re-authenticates.

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

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

@@ -56,6 +56,10 @@ export class AgrarmonitorService {
     return this.client;
   }
 
+  clearClient(): void {
+    this.client = null;
+  }
+
   async getStatus(): Promise<AgrarmonitorStatusDto> {
     try {
       const client = await this.getClient();