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/ValidationProblemResponseFactory.cs

@@ -0,0 +1,18 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+
+namespace MicCheck.Api.Common.Validation;
+
+public static class ValidationProblemResponseFactory
+{
+    public static IActionResult Create(ModelStateDictionary modelState)
+    {
+        var errors = modelState
+            .Where(e => e.Value?.Errors.Count > 0)
+            .ToDictionary(
+                kvp => kvp.Key,
+                kvp => kvp.Value!.Errors.Select(e => e.ErrorMessage).ToArray());
+
+        return new UnprocessableEntityObjectResult(new { errors });
+    }
+}