Move FeatureUsage code into MicCheck.Api.Features.Usage namespace

Groups usage-tracking (metrics, background flush, query service,
controller, entity) under its own namespace/folder, updates all
consumers and the EF migration snapshot's CLR type strings to match.

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

@@ -1,4 +1,5 @@
 using MicCheck.Api.Data;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Identities;
 using MicCheck.Api.Segments;
 using Microsoft.EntityFrameworkCore;