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/FeatureUsageController.cs

@@ -0,0 +1,29 @@
+using MicCheck.Api.Common.Security.Authorization;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.RateLimiting;
+using Microsoft.Extensions.Caching.Memory;
+
+namespace MicCheck.Api.Features.Usage;
+
+[ApiController]
+[Route("api/v1/environment/{environmentId}/usage")]
+[Authorize(Policy = AuthorizationPolicies.AdminApiAccess)]
+[EnableRateLimiting("AdminApi")]
+public class FeatureUsageController(FeatureUsageQueryService queryService, IMemoryCache cache) : ControllerBase
+{
+    [HttpGet]
+    public async Task<ActionResult<DashboardUsageResponse>> GetDashboardUsage(int environmentId, [FromQuery] int days = 14, CancellationToken ct = default)
+    {
+        days = Math.Clamp(days, 1, 90);
+        var cacheKey = $"usage-dashboard:{environmentId}:{days}";
+
+        if (cache.TryGetValue(cacheKey, out DashboardUsageResponse? cached))
+            return Ok(cached);
+
+        var result = await queryService.GetDashboardUsageAsync(environmentId, days, ct);
+
+        cache.Set(cacheKey, result, TimeSpan.FromSeconds(180));
+        return Ok(result);
+    }
+}