Add-to-carts now store their amount and cart reference

Each add-to-cart now stores what the quantity added is worth and a reference to the cart it went into.

• Amount and currency. The line's total for the quantity added, kept in the currency's minor units. If the amount is missing or invalid, the add is still counted without it.
• Cart reference. Shopify's cart token. The access key that follows it in the cart cookie is never sent or stored.
• Where it comes from. The Shopify pixel for adds on your pages, and the assistant's own cart for adds in the panel, when its cart result gives a price and a currency.

The amounts are stored but not shown yet: no report displays them. If you already installed the pixel, paste the new code from Site settings > Installation to send them. Nothing else changes: the pixel still sends no customer field, still honors the shopper's objection, and the on/off switch works as before.

Erasing a conversation also clears the cart reference on its adds. Expiry on the normal retention schedule keeps it.