Update broadcast

curl -X PATCH 'https://api.sndr.sh/v1/broadcasts/bca_9b3e6d12-a40' \
  -H "Authorization: Bearer $SNDR_API_KEY" \
  -H 'Content-Type: application/json' \
  -d '{"subject": "June product update: highlights"}'

{
  "id":               "bca_9b3e6d12-a40",
  "organization_id":  "org_8d41c7a2-3f6",
  "name":             "June newsletter",
  "group_id":         "grp_4d8a0c5e-1f7",
  "from_email":       "hello@acme.com",
  "subject":          "June product update: highlights",
  "html":             "<h1>June update</h1><p>Here is what is new.</p>",
  "status":           "draft",
  "total_recipients": 0,
  "sent_count":       0,
  "failed_count":     0,
  "created_at":       "2026-05-28T14:00:00Z"
}