fix: add missing BaseModel import for LetzshopExportRequest

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/vendors.py

@@ -11,6 +11,7 @@ Architecture Notes:
 import logging
 
 from fastapi import APIRouter, Body, Depends, Path, Query
+from pydantic import BaseModel
 from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api