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.

tests/api/MicCheck.Api.Tests.Unit/Identities/IdentitiesControllerTests.cs

@@ -2,6 +2,7 @@ using System.Security.Claims;
 using MicCheck.Api.Data;
 using MicCheck.Api.Environments;
 using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Identities;
 using MicCheck.Api.Projects;
 using MicCheck.Api.Segments;