One endpoint per portal account. The token in the path identifies the account and is
rotatable from the CRM.

Property Finder additionally signs each delivery: a hex HMAC-SHA256 of the raw body
in x-signature, keyed with the secret given at subscription time. When both a
signature and a stored secret are present the signature must match or the delivery
is refused; the unguessable URL is the fallback for subscriptions made without one.

Portals expect an answer within about five seconds, so the delivery is stored and
acknowledged first and the lead is filed immediately after.