Create a stream (optionally with its first schema version and sources)

A Stream collects Submissions from many Forms into one shared shape. Pass schema to define that shape up front, or omit it and pass sources — the first source's form then provides version 1 (copied from its published schema, its inferred draft, or the fields seen in recent submissions) with an identity mapping, so { name, sources: [{ form_id }] } is a complete request.