# description
POST /api/broadcasts/:id/send — 202 with the broadcast's new state: 'scheduled' when a future scheduledAt was given, else 'sending' (fan-out continues in the background; poll GET /api/broadcasts/:id for progress).

# properties.scheduledAt.description
When a scheduled broadcast will send. Present only when status is 'scheduled', in Unix epoch milliseconds (UTC).

# properties.audienceCount.description
Audience size snapshotted at this moment.