Extend the submission contract to accept a `systemInfo` object and persist it with each submission, including deep-copy support for `extra` metadata.
Also update client-facing docs and HTTP examples (JSON and multipart) and document that the schema is available at `GET /api/schema`, so clients can reliably implement the updated payload format.feat(api): support optional systemInfo in submissions
Extend the submission contract to accept a `systemInfo` object and persist it with each submission, including deep-copy support for `extra` metadata.
Also update client-facing docs and HTTP examples (JSON and multipart) and document that the schema is available at `GET /api/schema`, so clients can reliably implement the updated payload format.
