API to list scheduled broadcasts

There's no way to fetch pending scheduled broadcasts via the API. GET /v1/message/broadcast and /v1/message/broadcasts both 404. The dashboard (Bulk Messages → Scheduled) shows them, but the API can't.

Impact: when our broadcast queue stalled, we couldn't enumerate what was pending or reconcile after recovery — we had to work blind. A GET that returns pending broadcasts (id, chat, message, scheduled_at, status) would make the API self-sufficient for anyone automating reminders.