feat(webhooks): HMAC/bearer/Docker/Standard Webhooks delivery modes #1078

Job	Run time
v3 reference seller — pytest (respx-mocked upstream)	43s
Validate conventional commit format	7s
Downstream import smoke (representative consumer symbols)	29s
AdCP storyboard runner — examples/seller_agent.py	1m 28s
AdCP storyboard runner — v3 reference seller (translator)	1m 22s
Postgres conformance tests (Postgres 16)	1m 5s
Test Python 3.12	3m 26s
Test Python 3.13	3m 8s
Validate schemas are up-to-date	3m 35s
Test Python 3.11	3m 27s
Test Python 3.10	3m 29s
	22m 19s

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