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%.
