Inbound-call hook for the warm-transfer phone number.

Registered as the phone number's inbound_webhook_url. Retell hits this before answering an inbound call; we match by (from_number, to_number) against the Redis stash WarmTransferCallAction wrote just before the cold transfer, and return the briefing + destination as dynamic_variables so the warm-transfer flow starts with {{transfer_number}} / {{briefing}} populated. Response shape follows Retell's inbound-webhook contract: {"call_inbound": {"dynamic_variables": {...}}}.