Save a file to the project drive

The request body is the file. Send Content-Length and x-content-sha256 (the lower-case hex SHA-256 of the body); Content-Type is stored with it (application/octet-stream: from the extension). Every write is a new version; the same bytes and type already current at the path answer 200 without one. Large files stream through multipart storage. Paths are relative to the project space root (tasks/t_ab12/evidence/healthz.png).