Marketplace Message Relay

A seller packs and posts a parcel, so they need a way to reach the buyer when something goes wrong with the delivery. They do not need the buyer's email address to do it — that is the one contact detail that lets a marketplace's customer be taken off the marketplace, and Spree now withholds it from the seller order (6.0-seller-product-submission.md).

Today that leaves the phone on the shipping address as the only contact route. This plan adds the second: a relayed message between buyer and seller where neither learns the other's real address.

Rails ships the machinery. Action Mailbox is already required by the starter app and unused; has_secure_token is the same primitive Spree uses for cart tokens, invitations and digital links.

Tracked in Linear as V-3615.