WebhookResponse now carries a `warnings: list[str]` field. When the
subscription's URL starts with http://, an `insecure_url` advisory is
surfaced on every GET/CREATE without blocking the request. HMAC still
detects tampering regardless of transport — only read-confidentiality
is lost over plaintext — and test/dev environments without TLS stay
usable.
Matches the operator-trust posture already established by DA-06
(admin-on-admin protection is out of scope). The alternative — hard
rejection at admin time — was considered and declined; warning-plus-
visibility is the right shape.
THREAT_MODEL WH-03 accepted risk registered; revisit triggers are
multi-admin delegation, a regulated customer, or an operator ticket
asking for a DECNET_WEBHOOK_REQUIRE_HTTPS enforcement knob.