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-frontend/src/api/settings.ts

@@ -225,4 +225,6 @@ export const agrarmonitorApi = {
     api.post<AgrarmonitorPollingResult>('/api/agrarmonitor/run-polling').then((r) => r.data),
   processUploads: () =>
     api.post<AgrarmonitorPollingResult>('/api/agrarmonitor/process-uploads').then((r) => r.data),
+  syncCorrespondents: () =>
+    api.post<{ total: number; matched: number; unmatched: number }>('/api/agrarmonitor/sync-correspondents').then((r) => r.data),
 };