Python stdlib ThreadingHTTPServer that accepts any POST path, optionally
verifies HMAC against --secret / $DECNET_MOCK_SECRET, and pretty-prints
each delivery with topic / event-id / signature status. Pass --fail 503
to exercise the worker's retry/backoff path.
Point a webhook at http://localhost:8765/ and you'll see every delivery
land with color-coded HMAC OK / MISMATCH / UNVERIFIED badges. No deps.
