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/Webhooks/WebhookResponseTests.cs

@@ -0,0 +1,110 @@
+using MicCheck.Api.Webhooks;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Webhooks;
+
+[TestFixture]
+public class WebhookResponseTests
+{
+    [Test]
+    public void WhenMappingAnEnvironmentScopedWebhook_ThenScopeIsSerializedAsItsName()
+    {
+        var webhook = new Webhook
+        {
+            Id = 1,
+            Url = "https://example.com/hook",
+            Secret = "s3cr3t",
+            Scope = WebhookScope.Environment,
+            EnvironmentId = 5,
+            Enabled = true,
+            CreatedAt = DateTimeOffset.UtcNow
+        };
+
+        var response = WebhookResponse.From(webhook);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Url, Is.EqualTo("https://example.com/hook"));
+        Assert.That(response.Secret, Is.EqualTo("s3cr3t"));
+        Assert.That(response.Scope, Is.EqualTo("Environment"));
+        Assert.That(response.Enabled, Is.True);
+        Assert.That(response.EnvironmentId, Is.EqualTo(5));
+        Assert.That(response.OrganizationId, Is.Null);
+    }
+
+    [Test]
+    public void WhenMappingAnOrganizationScopedWebhook_ThenScopeIsSerializedAsItsName()
+    {
+        var webhook = new Webhook
+        {
+            Id = 2,
+            Url = "https://example.com/hook",
+            Scope = WebhookScope.Organization,
+            OrganizationId = 9,
+            Enabled = false,
+            CreatedAt = DateTimeOffset.UtcNow
+        };
+
+        var response = WebhookResponse.From(webhook);
+
+        Assert.That(response.Scope, Is.EqualTo("Organization"));
+        Assert.That(response.OrganizationId, Is.EqualTo(9));
+        Assert.That(response.EnvironmentId, Is.Null);
+    }
+}
+
+[TestFixture]
+public class WebhookDeliveryLogResponseTests
+{
+    [Test]
+    public void WhenMappingASuccessfulDelivery_ThenAllFieldsAreCopied()
+    {
+        var log = new WebhookDeliveryLog
+        {
+            Id = 1,
+            WebhookId = 2,
+            EventType = "FLAG_UPDATED",
+            PayloadJson = "{}",
+            ResponseStatusCode = 200,
+            ResponseBody = "ok",
+            Success = true,
+            AttemptNumber = 1,
+            AttemptedAt = DateTimeOffset.UtcNow,
+            Duration = TimeSpan.FromMilliseconds(42)
+        };
+
+        var response = WebhookDeliveryLogResponse.From(log);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.WebhookId, Is.EqualTo(2));
+        Assert.That(response.EventType, Is.EqualTo("FLAG_UPDATED"));
+        Assert.That(response.Success, Is.True);
+        Assert.That(response.ResponseStatusCode, Is.EqualTo(200));
+        Assert.That(response.ResponseBody, Is.EqualTo("ok"));
+        Assert.That(response.ErrorMessage, Is.Null);
+        Assert.That(response.AttemptNumber, Is.EqualTo(1));
+        Assert.That(response.Duration, Is.EqualTo(TimeSpan.FromMilliseconds(42)));
+    }
+
+    [Test]
+    public void WhenMappingAFailedDelivery_ThenErrorMessageIsCopied()
+    {
+        var log = new WebhookDeliveryLog
+        {
+            Id = 1,
+            WebhookId = 2,
+            EventType = "FLAG_UPDATED",
+            PayloadJson = "{}",
+            Success = false,
+            ErrorMessage = "Connection refused",
+            AttemptNumber = 2,
+            AttemptedAt = DateTimeOffset.UtcNow,
+            Duration = TimeSpan.Zero
+        };
+
+        var response = WebhookDeliveryLogResponse.From(log);
+
+        Assert.That(response.Success, Is.False);
+        Assert.That(response.ErrorMessage, Is.EqualTo("Connection refused"));
+        Assert.That(response.ResponseStatusCode, Is.Null);
+    }
+}