Create an envelope — and send it, with `send: true`

The PDF is sourceSha256 + sourceFilename (from POST /source), or an Archivio document: documentId alone signs its current version, and its signed copy becomes the document’s next version once everybody has signed. With both, the staged PDF must be that document’s current version (FIRME_SOURCE_NOT_DOCUMENT). Signers sign in order (routing: "sequential") or all at once; an address that is a member’s login is bound to that member, who signs in the app. inPerson signers draw their signature on a member’s phone. Without send the envelope is a draft nobody hears about. Signature boxes (signers[].fields, at most one per signer, type: "signature") are in PDF points (1/72 inch) of the page’s own user space: page from 1, x/y the box’s BOTTOM-LEFT corner, origin at the bottom-left of the page. A signer with no box signs invisibly, except the first signature on a document, which is drawn as a vertical strip along the left edge of page 1; defaultSignatureField: true puts a box at the foot of the last page instead.