Upload Excel Template

Upload and save an Excel template for future use. Extracts schema and stores file in S3. Supports both authenticated users and guest mode.

Args: file: Excel file (.xlsx or .xls) description: Optional description of the template current_user: Authenticated user (optional for guest mode)

Returns: { "template_id": "uuid", "name": "filename.xlsx", "detected_fields": [...], "s3_key": "excel_templates/..." }