Send a broadcast to its audience

Snapshot the broadcast's audience (the list's subscribed, non-dormant members, narrowed by its segment and limit, most engaged first when audience.order is engaged) and send one message to each in the background. Every message goes through the normal send checks: verified domain, the marketing plan (integration-trial workspaces may only send to the account email), suppressions, unsubscribes, sender reputation, and daily and monthly limits, and carries one-click List-Unsubscribe headers that remove the recipient from this list only (the unsubscribe page also offers leaving all marketing email; receipts and other transactional mail are never blocked) and the postal address from newsletter settings. With tracking on, links are rewritten to signed SendHQ redirects and an open pixel is added; the unsubscribe link is never rewritten. Members that cannot be mailed are recorded as skipped. Each contact is sent at most once per broadcast. When a limit or a reputation pause stops delivery, the broadcast keeps sending and resumes after nextAttemptAt. A scheduled draft returns 409 until it is unscheduled. Returns 202.