Upload a new version of a script.

The Writers Lab's "upload a new version", through its core: the next version number, parsed and analysed in the background, with the previous version's episode overrides copied forward. Earlier versions are kept. A new version can come from a file (a PDF, a Final Draft .fdx, or a Fountain or plain-text script) instead of its text. The file is read as a source of the script's project: fileUrl (a link to the file) adds it and answers waiting with its sourceId; check that source until it is READY, then send sourceId and its text becomes the new version. A source that is already READY becomes the version at once, and the same source makes its version once (sending it again answers that version). A script in no project has nowhere to keep the file: send its text instead. A source that could not be read is refused (400) with its reason. Refused with 403 plan_limit_reached on the free plan (one version per script) or past the page allowance; 404 script_not_found for a script that is not yours, source_not_found for a source that is not. Free. Requires Idempotency-Key.