terms of service flow for first reply

Require user to accept terms of service for first message received by the service from a given sender. (Is this possible to check unique devices for inreach?)

Require reply message, "accept", how to remember, maybe use database?

The issue names no files, tests, or entry points. Start by locating the handler for the first message from a sender, then determine how sender acceptance state can be persisted and tested; done means an unaccepted sender receives the required reply and accepted senders can continue.

Written by the indexing model from the issue text.