Create scratchpad item

curl -X POST https://postiv.ai/api/public/scratchpad \
  -H "Authorization: Bearer pk_postiv_..." \
  -H "Content-Type: application/json" \
  -d '{
    "kind": "note",
    "content": "Three things we learned from launch week..."
  }'

{
  "success": true,
  "item": {
    "id": "sp2...",
    "kind": "url",
    "url": "https://example.com/article",
    "status": "fresh",
    "createdAt": "2026-07-19T09:00:00.000Z"
  }
}