Add feature-usage tracking and dashboard charts

Instruments FeatureEvaluationService with System.Diagnostics.Metrics (Counter + IMeterFactory). Accumulates per-feature/day evaluation counts in-process, flushed every 30s to a new FeatureUsageDaily Postgres table via PostgreSQL ON CONFLICT upsert. Caches dashboard query results for 180s in IMemoryCache. New GET /api/v1/environments/{id}/usage endpoint returns top-10 features last day and 14-day per-day breakdown. Admin dashboard shows two ApexCharts bar charts: top features and evaluations by day with per-flag hover tooltip.

tests/api/MicCheck.Api.Tests.Unit/Features/FeatureUsageMetricsTests.cs

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+using System.Diagnostics.Metrics;
+using MicCheck.Api.Features;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeatureUsageMetricsTests
+{
+    private FeatureUsageMetrics _metrics = null!;
+
+    [SetUp]
+    public void SetUp()
+    {
+        var meterFactory = new Mock<IMeterFactory>();
+        meterFactory.Setup(f => f.Create(It.IsAny<MeterOptions>())).Returns(new Meter("test"));
+        _metrics = new FeatureUsageMetrics(meterFactory.Object);
+    }
+
+    [TearDown]
+    public void TearDown() => _metrics.Dispose();
+
+    [Test]
+    public void WhenRecordingEvaluations_ThenAccumulatorIncrementsByFeatureAndEnvironment()
+    {
+        _metrics.RecordEvaluation(environmentId: 1, featureId: 10, featureName: "dark_mode");
+        _metrics.RecordEvaluation(environmentId: 1, featureId: 10, featureName: "dark_mode");
+        _metrics.RecordEvaluation(environmentId: 1, featureId: 20, featureName: "beta");
+        _metrics.RecordEvaluation(environmentId: 2, featureId: 10, featureName: "dark_mode");
+
+        var drained = _metrics.DrainAccumulated();
+
+        Assert.That(drained, Has.Count.EqualTo(3));
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        Assert.That(drained[new FeatureUsageBucketKey(1, 10, "dark_mode", today)], Is.EqualTo(2));
+        Assert.That(drained[new FeatureUsageBucketKey(1, 20, "beta", today)], Is.EqualTo(1));
+        Assert.That(drained[new FeatureUsageBucketKey(2, 10, "dark_mode", today)], Is.EqualTo(1));
+    }
+
+    [Test]
+    public void WhenDrainCalled_ThenAccumulatorIsResetToEmpty()
+    {
+        _metrics.RecordEvaluation(environmentId: 1, featureId: 10, featureName: "dark_mode");
+
+        _metrics.DrainAccumulated();
+        var secondDrain = _metrics.DrainAccumulated();
+
+        Assert.That(secondDrain, Is.Empty);
+    }
+
+    [Test]
+    public void WhenNothingRecorded_ThenDrainReturnsEmptyDictionary()
+    {
+        var drained = _metrics.DrainAccumulated();
+
+        Assert.That(drained, Is.Empty);
+    }
+
+    [Test]
+    public void WhenRecordingConcurrently_ThenCountsAreAccurateAndNoDataLost()
+    {
+        const int threads = 8;
+        const int recordsPerThread = 100;
+
+        var tasks = Enumerable.Range(0, threads).Select(_ =>
+            Task.Run(() =>
+            {
+                for (var i = 0; i < recordsPerThread; i++)
+                    _metrics.RecordEvaluation(environmentId: 1, featureId: 1, featureName: "concurrent_flag");
+            })).ToArray();
+
+        Task.WaitAll(tasks);
+
+        var drained = _metrics.DrainAccumulated();
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+
+        Assert.That(drained[new FeatureUsageBucketKey(1, 1, "concurrent_flag", today)], Is.EqualTo(threads * recordsPerThread));
+    }
+}