Remove FluentValidation; validator/DI cleanup (#7)

## Summary
- Remove FluentValidation dependency; replace with plain IModelValidator<T>/ValidationResult pattern (ported from BuyEngine's Guard/validation approach), wired via a ModelValidationActionFilter
- Move FeatureUsage code into MicCheck.Api.Features.Usage namespace
- Convert logic-free data classes to records per CLAUDE.md (WebhookEvent, FeatureStateResult, ProjectPermissionRequirement, AuditLog, Tag, FeatureUsageDaily)
- Extract IAuditService interface, drop virtual-method mock seam on AuditService
- Split Program.cs DI registrations into per-namespace DependencyRegistration classes

## Test plan
- [x] dotnet build (API + tests) clean
- [x] dotnet test MicCheck.Api.Tests.Unit — 646/646 passing
- [x] pre-push hook (full solution build/test + admin build) passed

Co-authored-by: miccheck-ci <ci@miccheck.local>
Reviewed-on: #7

tests/api/MicCheck.Api.Tests.Unit/Features/FeatureEvaluationServiceTests.cs

@@ -1,6 +1,7 @@
 using System.Diagnostics.Metrics;
 using MicCheck.Api.Data;
 using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Identities;
 using MicCheck.Api.Segments;
 using MicCheck.Api.Tests.Unit.TestSupport;

tests/api/MicCheck.Api.Tests.Unit/Features/FeatureServiceTests.cs

@@ -45,7 +45,7 @@ public class FeatureServiceTests
         _db.SetupDbSetWithGeneratedIds(c => c.Tags, _tags);
 
         var webhookQueue = new MicCheck.Api.Webhooks.WebhookQueue();
-        var auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        var auditService = new Mock<IAuditService>();
         auditService.Setup(a => a.RecordAsync(
             It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
             It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),

tests/api/MicCheck.Api.Tests.Unit/Features/FeatureStateServiceTests.cs

@@ -43,7 +43,7 @@ public class FeatureStateServiceTests
         _db.SetupDbSetWithGeneratedIds(c => c.FeatureStates, _featureStates);
 
         var webhookQueue = new WebhookQueue();
-        var auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        var auditService = new Mock<IAuditService>();
         auditService.Setup(a => a.RecordAsync(
             It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
             It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),
@@ -154,7 +154,7 @@ public class FeatureStateServiceTests
         var state = AddState();
         var queue = new WebhookQueue();
 
-        var auditService = new Mock<AuditService>(_db.Object, null!, queue);
+        var auditService = new Mock<IAuditService>();
         auditService.Setup(a => a.RecordAsync(
             It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
             It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),

tests/api/MicCheck.Api.Tests.Unit/Features/FeaturesControllerTests.cs

@@ -41,7 +41,7 @@ public class FeaturesControllerTests
         _db.SetupDbSetWithGeneratedIds(c => c.Tags, _tags);
 
         var webhookQueue = new MicCheck.Api.Webhooks.WebhookQueue();
-        var auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        var auditService = new Mock<IAuditService>();
         auditService.Setup(a => a.RecordAsync(
             It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
             It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),

tests/api/MicCheck.Api.Tests.Unit/Features/FlagsApiIntegrationTests.cs

@@ -2,6 +2,7 @@ using System.Security.Claims;
 using MicCheck.Api.Data;
 using MicCheck.Api.Environments;
 using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Identities;
 using MicCheck.Api.Projects;
 using MicCheck.Api.Segments;

tests/api/MicCheck.Api.Tests.Unit/Features/Usage/FeatureUsageControllerTests.cs

@@ -1,12 +1,12 @@
 using MicCheck.Api.Data;
-using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Tests.Unit.TestSupport;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.Extensions.Caching.Memory;
 using Moq;
 using NUnit.Framework;
 
-namespace MicCheck.Api.Tests.Unit.Features;
+namespace MicCheck.Api.Tests.Unit.Features.Usage;
 
 [TestFixture]
 public class FeatureUsageControllerTests

tests/api/MicCheck.Api.Tests.Unit/Features/Usage/FeatureUsageMetricsTests.cs

@@ -1,9 +1,9 @@
 using System.Diagnostics.Metrics;
-using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using Moq;
 using NUnit.Framework;
 
-namespace MicCheck.Api.Tests.Unit.Features;
+namespace MicCheck.Api.Tests.Unit.Features.Usage;
 
 [TestFixture]
 public class FeatureUsageMetricsTests

tests/api/MicCheck.Api.Tests.Unit/Features/Usage/FeatureUsageQueryServiceTests.cs

@@ -1,10 +1,10 @@
 using MicCheck.Api.Data;
-using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Tests.Unit.TestSupport;
 using Moq;
 using NUnit.Framework;
 
-namespace MicCheck.Api.Tests.Unit.Features;
+namespace MicCheck.Api.Tests.Unit.Features.Usage;
 
 [TestFixture]
 public class FeatureUsageQueryServiceTests