List Files

List files under a given S3 prefix (file tree).

Requires platform admin role.

Args: prefix: S3 prefix to list files under (default: "" for all files) max_keys: Maximum number of keys to return (default: 1000)

Returns: ListFilesResponse with list of file keys, prefix, and count

Raises: HTTPException: 403 if not platform admin, 500 for S3 errors