Add file_count and files_total_size to item API responses, computed
via batch query on item_files table (no migration needed).
- Add BatchGetFileStats() to audit_queries.go (follows BatchCheckBOM pattern)
- Add file stats to ItemResponse, HandleListItems, HandleGetItem, HandleGetItemByUUID
- Add 'Files' column to ItemTable (default visible in vertical mode)
- Add has_files computed field to audit completeness scoring (weight 1 for manufactured)