Inbound message

Notification of free-form content the user sends without linking it to a specific suggestion, within the `ttl_events` window of the last message sent to that number. If the window already expired, the event is **discarded** and not notified. The root `message_id` identifies the **inbound** message (not the outbound send); `data.related_message_id` points to the `message_id` of the most recent outbound message the conversation is associated with. The `data.message` object carries the canonical content and its shape depends on `type`: `TEXT` (`text`), `FILE` (`url`, `mime_type`, `file_name` — the file is already re-hosted in Chat Bridge storage, not the user's original), or `LOCATION` (`latitude`, `longitude`).