fix: export job records display shows files instead of mixed metrics

Previously export jobs showed "3/23" (languages/products) which was confusing.
Now shows "3/3" (files succeeded / files processed) for consistency.

- Updated log_export to track files_processed/succeeded/failed
- Products count stored in error_details for reference
- Updated documentation to explain records column meaning per job type

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/admin/vendors.py

@@ -445,15 +445,17 @@ def export_vendor_products_letzshop_to_folder(
             })
 
     # Log the export operation via service
+    # For exports, track files (languages) as the unit of work
     completed_at = datetime.now(UTC)
     letzshop_export_service.log_export(
         db=db,
         vendor_id=vendor.id,
         started_at=started_at,
         completed_at=completed_at,
-        records_processed=total_records,
-        languages_succeeded=len(languages) - failed_count,
-        languages_failed=failed_count,
+        files_processed=len(languages),
+        files_succeeded=len(languages) - failed_count,
+        files_failed=failed_count,
+        products_exported=total_records,
         triggered_by=f"admin:{current_admin.id}",
         error_details={"files": exported_files} if failed_count > 0 else None,
     )