Add a document to a project

Provide base64 content to store the file and queue reading, or omit content to prepare metadata for multipart upload; readJobId is null until original bytes arrive. A file with the name of one already in the folder becomes a replacement of its current original. Base64 content is limited to 20 MiB; multipart transport accepts up to 250 MiB. Replacement invalidates extraction. Poll getProjectFile until extraction is ready (or failed); a planning case (PSS/E RAW) is then studied at the project's POI on its own.