Deepen webhook ingestion module

[darthfork]

Summary

• add a webhook ingestion Module with an interface-level Accept path for signature validation, delivery dedupe, queue acceptance, sync dispatch, and status outcomes
• turn githubEventsHandler into an HTTP Adapter that reads request data and delegates to the ingestion Module
• add interface-level tests for invalid signatures, duplicate deliveries, delivery-store errors, sync dispatch, async enqueue, queue-full behavior, and HTTP header/body adaptation

TDD / Tests

• Added failing tests first in src/webhook_ingestion_test.go, then implemented the Module and Adapter
• Verified with: mise exec go@1.25.10 -- go test ./...

Notes

• The repo's .go-version is 1.25.10, while plain go on PATH reported 1.25.9 in this shell, so verification used mise exec go@1.25.10.