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/Features/Usage/FeatureUsageFlushBackgroundService.cs

@@ -0,0 +1,62 @@
+using System.Diagnostics.CodeAnalysis;
+using MicCheck.Api.Data;
+using Microsoft.EntityFrameworkCore;
+
+namespace MicCheck.Api.Features.Usage;
+
+[ExcludeFromCodeCoverage(Justification = "Timer-driven BackgroundService that issues raw SQL through a DI-scoped concrete MicCheckDbContext; exercising it cleanly requires a live DB, which CLAUDE.md disallows (no WebApplicationFactory/InMemory). DrainAccumulated's bucketing logic is covered by FeatureUsageMetricsTests.")]
+public class FeatureUsageFlushBackgroundService(
+    FeatureUsageMetrics metrics,
+    IServiceScopeFactory scopeFactory,
+    ILogger<FeatureUsageFlushBackgroundService> logger) : BackgroundService
+{
+    private static readonly TimeSpan FlushInterval = TimeSpan.FromSeconds(30);
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        using var timer = new PeriodicTimer(FlushInterval);
+        while (await timer.WaitForNextTickAsync(stoppingToken))
+        {
+            await FlushAsync(stoppingToken);
+        }
+    }
+
+    public override async Task StopAsync(CancellationToken cancellationToken)
+    {
+        await base.StopAsync(cancellationToken);
+        await FlushAsync(CancellationToken.None);
+    }
+
+    private async Task FlushAsync(CancellationToken ct)
+    {
+        var deltas = metrics.DrainAccumulated();
+        if (deltas.Count == 0)
+            return;
+
+        var updatedAt = DateTimeOffset.UtcNow;
+
+        try
+        {
+            await using var scope = scopeFactory.CreateAsyncScope();
+            var db = scope.ServiceProvider.GetRequiredService<MicCheckDbContext>();
+
+            foreach (var (key, count) in deltas)
+            {
+                await db.Database.ExecuteSqlAsync(
+                    $"""
+                    INSERT INTO "FeatureUsageDaily" ("EnvironmentId", "FeatureId", "FeatureName", "UsageDate", "Count", "UpdatedAt")
+                    VALUES ({key.EnvironmentId}, {key.FeatureId}, {key.FeatureName}, {key.UsageDate}, {count}, {updatedAt})
+                    ON CONFLICT ("EnvironmentId", "FeatureId", "UsageDate")
+                    DO UPDATE SET "Count" = "FeatureUsageDaily"."Count" + EXCLUDED."Count",
+                                  "FeatureName" = EXCLUDED."FeatureName",
+                                  "UpdatedAt" = EXCLUDED."UpdatedAt"
+                    """,
+                    ct);
+            }
+        }
+        catch (Exception ex)
+        {
+            logger.LogError(ex, "Failed to flush feature usage data ({BucketCount} buckets)", deltas.Count);
+        }
+    }
+}