Check file existence and get metadata headers

Lightweight existence check and metadata retrieval. Returns the same metadata as GET /{file_id} but as HTTP response headers and with no body — useful for existence checks and ETag-based conditional requests without transferring file content. Mirrors S3 HeadObject semantics. Headers returned: X-File-Id, X-File-Mime-Type, X-File-Size, X-File-Etag, X-File-Namespace, X-File-Filename, X-File-Created-At.