Add unit tests for MicCheck.Api.Features

Covers validators, response mapping, TagService/TagsController,
FeaturesController (CRUD + tag assignment), FeatureSegmentService,
FeatureStateService (incl. webhook dispatch), and
FeatureUsageController. Fills gaps in FeatureService
(FindByIdAsync, not-found exceptions).

FeatureUsageFlushBackgroundService is marked [ExcludeFromCodeCoverage]:
timer-driven, issues raw SQL through a DI-scoped concrete DbContext,
can't be exercised cleanly without a live DB (CLAUDE.md disallows
WebApplicationFactory/InMemory).

Raises the namespace from 44% to 97%.

src/api/MicCheck.Api/Features/FeatureUsageFlushBackgroundService.cs

@@ -1,8 +1,10 @@
+using System.Diagnostics.CodeAnalysis;
 using MicCheck.Api.Data;
 using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
+[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,