Add unit tests for Environments, Audit, and Webhooks namespaces (#6)

## 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

tests/api/MicCheck.Api.Tests.Unit/Environments/EnvironmentResponseTests.cs

@@ -0,0 +1,30 @@
+using NUnit.Framework;
+using AppEnvironment = MicCheck.Api.Environments.Environment;
+
+namespace MicCheck.Api.Tests.Unit.Environments;
+
+[TestFixture]
+public class EnvironmentResponseTests
+{
+    [Test]
+    public void WhenMappingAnEnvironment_ThenAllFieldsAreCopied()
+    {
+        var createdAt = DateTimeOffset.UtcNow;
+        var environment = new AppEnvironment
+        {
+            Id = 1,
+            Name = "Production",
+            ApiKey = "env-key-abc",
+            ProjectId = 2,
+            CreatedAt = createdAt
+        };
+
+        var response = MicCheck.Api.Environments.EnvironmentResponse.From(environment);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Name, Is.EqualTo("Production"));
+        Assert.That(response.ApiKey, Is.EqualTo("env-key-abc"));
+        Assert.That(response.ProjectId, Is.EqualTo(2));
+        Assert.That(response.CreatedAt, Is.EqualTo(createdAt));
+    }
+}