* feat(refund): add refund request flow
- tui modal composer with reason select + free-form message
(pkg/tui/refund.go)
- POST /api/v1/orders/{id}/refund-request posts to orders slack thread
- 15 min server side cooldown via Order.LastRefundRequestAt (429 +
retry-after)
- shared refundrequestreasons + can requestrefund hoisted to pkg/models
- slack message includes customer name, email, order total, quoted body
- tests over invalid-state, missing-message, and cooldown paths
* style(tui): run gofmt to fix formatting
