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/Projects/CreateProjectRequest.cs

@@ -1,14 +1,23 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Projects;
 
 public record CreateProjectRequest(string Name, int OrganizationId);
 
-public class CreateProjectRequestValidator : AbstractValidator<CreateProjectRequest>
+public class CreateProjectRequestValidator : IModelValidator<CreateProjectRequest>
 {
-    public CreateProjectRequestValidator()
+    public ValidationResult Validate(CreateProjectRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
-        RuleFor(x => x.OrganizationId).GreaterThan(0);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        if (model.OrganizationId <= 0)
+            result.AddError(nameof(model.OrganizationId), "'Organization Id' must be greater than 0.");
+
+        return result;
     }
 }