Hourtick Invoices API
Invoices through the workspace's own Stripe account (Stripe Connect). An admin connects Stripe in Settings; Stripe Invoicing issues, emails and collects each invoice; Hourtick takes no fee (Stripe's own fees apply). Lines come from billable time not invoiced yet, and sent time is locked as invoiced. Agents send on their own only inside an invoice mandate; otherwise the invoice waits for an admin's approval. Writes take an `idempotencyKey` (body) or `Idempotency-Key` header.