GET /v1/invoices/{id}/pdf — partial-parity #18

Streams the same PDF the customer downloads from /portal/{hash}/invoice/pdf (and the operator gets from /admin/bookings/{id}/invoice/pdf). The Payment id (in_X) maps to its parent booking; PDF content is per-booking, so multiple payments on the same booking return the same document.

application/pdf binary; attachment disposition so cc can stash the file or surface a download button.