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

src/api/MicCheck.Api/Common/Validation/ValidationResult.cs

@@ -0,0 +1,14 @@
+namespace MicCheck.Api.Common.Validation;
+
+public record ValidationError(string PropertyName, string Message);
+
+public class ValidationResult
+{
+    private readonly List<ValidationError> _errors = [];
+
+    public IReadOnlyList<ValidationError> Errors => _errors;
+    public bool IsValid => _errors.Count == 0;
+    public bool IsInvalid => _errors.Count > 0;
+
+    public void AddError(string propertyName, string message) => _errors.Add(new ValidationError(propertyName, message));
+}