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/Audit/AuditLogsControllerTests.cs

@@ -46,7 +46,7 @@ public class AuditLogsControllerTests
         _db.SetupDbSet(c => c.AuditLogs, _auditLogs);
         _db.SetupDbSet(c => c.Users, []);
 
-        var auditServiceMock = new Mock<AuditService>(_db.Object, null!, new MicCheck.Api.Webhooks.WebhookQueue());
+        var auditServiceMock = new Mock<IAuditService>();
         var auditService = auditServiceMock.Object;
 
         _controller = new AuditLogsController(