Submit a call

Records one thing that happened with a customer outside a Callens call — a message, an order, a note — from any system. One event per request. Callens recognises the customer from the identifiers in `customer` and stores the event on that customer. `event.content` is kept as descriptive text and is never parsed into fields. Sending the same `event.source` and `event.external_id` again returns the stored event and writes nothing, so the id must be the same on every retry of one event. Call this from a server you control and never from a browser or a mobile app: the response says whether an identifier already belongs to one of your customers and returns that customer's id, so a key with the `events` scope must not be readable by an anonymous client.