Fallback exhausted

Final notification of an SMS fallback chain: the backup SMS also failed, or it could not even be generated (for example, the `to` of the original send was a BSUID with no real phone number behind it). After this event the chain is over — no automatic retry follows. When the backup SMS failed, `message_id` is the identifier of the backup SMS (the `fallback_id`) and `origin_identifier` points back to the original WhatsApp `message_id`. When the SMS could not be generated, `message_id` is the original WhatsApp send identifier. Note: while the backup SMS is alive, its regular status events (`SENT`/`DELIVERED`/`FAILED`) arrive as **SMS** webhooks (see the SMS API documentation) carrying `origin_identifier` to correlate them with the WhatsApp send.