Consumers now depend on IAuditService instead of the concrete class,
so tests mock the interface (Mock<IAuditService>()) rather than
subclassing AuditService via virtual methods.
## Summary
- Add missing unit test coverage for the Environments, Audit, and Webhooks namespaces (raises them from ~0-62% to 91-100%)
- Exclude WebhookBackgroundService/WebhookRetryBackgroundService from coverage (require live DI/DB, disallowed by CLAUDE.md's no-InMemory/WebApplicationFactory rule)
## Test plan
- [x] `dotnet test` full suite passes (646/646)
- [x] Coverage report confirms Environments ~99.5%, Audit 100%, Webhooks ~91%
Reviewed-on: #6
