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/Admin/AdminApiIntegrationTests.cs

@@ -23,7 +23,7 @@ public class AdminApiIntegrationTests
     private List<FeatureState> _featureStates = null!;
     private List<AppEnvironment> _environments = null!;
     private List<Segment> _segments = null!;
-    private Mock<AuditService> _auditService = null!;
+    private Mock<IAuditService> _auditService = null!;
     private int _organizationId;
 
     [SetUp]
@@ -50,7 +50,7 @@ public class AdminApiIntegrationTests
         _db.SetupDbSetWithGeneratedIds(c => c.SegmentConditions, []);
 
         var webhookQueue = new WebhookQueue();
-        _auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        _auditService = new Mock<IAuditService>();
         _auditService.Setup(a => a.LogAsync(
             It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
             It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),