feat: detect and resolve duplicate correspondents in Agrarmonitor sync

- Detect duplicates after sync (same AgrarmonitorId, multiple correspondents)
- Auto-merge duplicates with identical names (delete empty, move docs to larger)
- Expose conflicts with different names for manual resolution
- New mergeCorrespondents endpoint + service method
- Conflict resolution modal in SettingsPage with radio selection per conflict
- deleteCorrespondent added to PaperlessService

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

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

@@ -180,6 +180,10 @@ export class PaperlessService {
     return response.data;
   }
 
+  async deleteCorrespondent(id: number): Promise<void> {
+    await this.client.delete(`/correspondents/${id}/`);
+  }
+
   async downloadDocument(id: number, type: 'original' | 'archive' = 'archive'): Promise<Buffer> {
     const endpoint = type === 'original'
       ? `/documents/${id}/download/`