Uploads a file and attaches it to the candidate as a document, returning the created document record.

Send the file as multipart/form-data with the field name document. On success the response includes the document's file metadata (file_name, file_url, file_size, creation_date) along with simplified position, acting_user, and candidate objects. Uploading a document also logs a "document added" company activity entry and updates the candidate's updated_date timestamp.