A product title containing & < > " ' silently breaks VK purchases: VK
HTML-escapes the character in its order_status_change payment-callback echo
(e.g. " -> ") but signs the notification over a different form, so the
gateway's signature check no longer matches ("vk callback: bad signature")
and the purchase never confirms — while get_item (whose request carries no
title) still passes.
validateProduct now rejects those characters at the admin catalog editor —
the single write path for CreateProduct/UpdateProduct/SetProductActive — so a
title can't reintroduce the break. Reject, not transform: the operator retypes
a clean title (« » guillemets, not straight quotes), so the stored value stays
identical everywhere (storefront, wallet, VK box).
Docs: docs/PAYMENTS.md §7 + agent field notes; Go Doc on validateProduct.
Test: validateProduct HTML-special-char cases. No wire/schema change.
Ilia Denisov