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/agrarmonitor/agrarmonitor.controller.ts

@@ -56,4 +56,11 @@ export class AgrarmonitorController {
   async syncCorrespondents() {
     return this.pollingService.syncCorrespondentIds();
   }
+
+  @Post('merge-correspondents')
+  @HttpCode(200)
+  @RequirePermissions(Permission.MANAGE_SETTINGS)
+  async mergeCorrespondents(@Body() body: { keepId: number; deleteId: number }) {
+    return this.pollingService.mergeCorrespondents(body.keepId, body.deleteId);
+  }
 }