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/Features/Usage/FeatureUsageQueryServiceTests.cs

@@ -0,0 +1,135 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Features.Usage;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features.Usage;
+
+[TestFixture]
+public class FeatureUsageQueryServiceTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<FeatureUsageDaily> _usage = null!;
+    private FeatureUsageQueryService _service = null!;
+    private const int EnvironmentId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+        _usage = [];
+        _db.SetupDbSet(c => c.FeatureUsageDaily, _usage);
+        _service = new FeatureUsageQueryService(_db.Object);
+    }
+
+    private void SeedUsage(int environmentId, int featureId, string featureName, DateOnly date, long count)
+    {
+        _usage.Add(new FeatureUsageDaily
+        {
+            EnvironmentId = environmentId,
+            FeatureId = featureId,
+            FeatureName = featureName,
+            UsageDate = date,
+            Count = count,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+    }
+
+    [Test]
+    public async Task WhenNoUsageData_ThenEmptyCollectionsAreReturned()
+    {
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 14);
+
+        Assert.That(result.TopFeaturesLastDay, Is.Empty);
+        Assert.That(result.DailyUsage, Is.Empty);
+    }
+
+    [Test]
+    public async Task WhenMultipleFeaturesEvaluatedToday_ThenTopFeaturesLastDayOrdersByCountDescending()
+    {
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        SeedUsage(EnvironmentId, featureId: 1, "feature_a", today, count: 50);
+        SeedUsage(EnvironmentId, featureId: 2, "feature_b", today, count: 200);
+        SeedUsage(EnvironmentId, featureId: 3, "feature_c", today, count: 10);
+
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 1);
+
+        Assert.That(result.TopFeaturesLastDay, Has.Count.EqualTo(3));
+        Assert.That(result.TopFeaturesLastDay[0].FeatureName, Is.EqualTo("feature_b"));
+        Assert.That(result.TopFeaturesLastDay[0].Count, Is.EqualTo(200));
+        Assert.That(result.TopFeaturesLastDay[1].FeatureName, Is.EqualTo("feature_a"));
+        Assert.That(result.TopFeaturesLastDay[2].FeatureName, Is.EqualTo("feature_c"));
+    }
+
+    [Test]
+    public async Task WhenMoreThanTenFeatures_ThenTopFeaturesLastDayLimitsToTen()
+    {
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        for (var i = 1; i <= 15; i++)
+            SeedUsage(EnvironmentId, featureId: i, $"feature_{i}", today, count: i * 10L);
+
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 1);
+
+        Assert.That(result.TopFeaturesLastDay, Has.Count.EqualTo(10));
+    }
+
+    [Test]
+    public async Task WhenUsageSpansMultipleDays_ThenDailyUsageGroupsByDateAscending()
+    {
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        SeedUsage(EnvironmentId, featureId: 1, "flag", today.AddDays(-2), count: 30);
+        SeedUsage(EnvironmentId, featureId: 1, "flag", today.AddDays(-1), count: 50);
+        SeedUsage(EnvironmentId, featureId: 1, "flag", today, count: 20);
+
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 7);
+
+        Assert.That(result.DailyUsage, Has.Count.EqualTo(3));
+        Assert.That(result.DailyUsage[0].Date, Is.EqualTo(today.AddDays(-2)));
+        Assert.That(result.DailyUsage[0].TotalCount, Is.EqualTo(30));
+        Assert.That(result.DailyUsage[1].TotalCount, Is.EqualTo(50));
+        Assert.That(result.DailyUsage[2].TotalCount, Is.EqualTo(20));
+    }
+
+    [Test]
+    public async Task WhenHoveringADay_ThenDailyUsageIncludesPerFeatureBreakdownOrderedByCount()
+    {
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        SeedUsage(EnvironmentId, featureId: 1, "feature_a", today, count: 10);
+        SeedUsage(EnvironmentId, featureId: 2, "feature_b", today, count: 40);
+
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 1);
+
+        var day = result.DailyUsage.Single();
+        Assert.That(day.TotalCount, Is.EqualTo(50));
+        Assert.That(day.Features, Has.Count.EqualTo(2));
+        Assert.That(day.Features[0].FeatureName, Is.EqualTo("feature_b"));
+        Assert.That(day.Features[0].Count, Is.EqualTo(40));
+        Assert.That(day.Features[1].FeatureName, Is.EqualTo("feature_a"));
+    }
+
+    [Test]
+    public async Task WhenUsageExistsOutsideDaysWindow_ThenOldDataIsExcluded()
+    {
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        SeedUsage(EnvironmentId, featureId: 1, "flag", today.AddDays(-30), count: 999);
+        SeedUsage(EnvironmentId, featureId: 1, "flag", today.AddDays(-3), count: 5);
+
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 7);
+
+        Assert.That(result.DailyUsage, Has.Count.EqualTo(1));
+        Assert.That(result.DailyUsage[0].TotalCount, Is.EqualTo(5));
+    }
+
+    [Test]
+    public async Task WhenOtherEnvironmentHasUsage_ThenItIsNotIncludedInResults()
+    {
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        SeedUsage(environmentId: 99, featureId: 1, "flag", today, count: 100);
+
+        var result = await _service.GetDashboardUsageAsync(EnvironmentId, days: 7);
+
+        Assert.That(result.TopFeaturesLastDay, Is.Empty);
+        Assert.That(result.DailyUsage, Is.Empty);
+    }
+}