curl --request POST \
  --url https://app.solya.app/api/rebalance-plans/{id}/items \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "quantity": 5,
  "sizeId": "sz-uuid-42",
  "variantId": "var-uuid-nike-air"
}
'

{
  "itemId": "rbi-uuid-003"
}