Function start 

pub async fn start(
    db: &Db,
    form: FormHandle,
    actor: Actor,
    spec: RunSpec,
    source: ByteStream,
) -> Result<i64, RunError>

Starts a run: records it, detaches the work and returns the run’s id for find to poll.

source is the file’s bytes; actor is who the rows are written as, on every record and audit entry. The spec is checked against the form first, so a mistaken screen fails here and no run row is left behind.