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/Data/MicCheckDbContext.cs

@@ -2,6 +2,7 @@ using MicCheck.Api.Common.Security.ApiKeys;
 using MicCheck.Api.Audit;
 using MicCheck.Api.Common.Security.Authorization;
 using MicCheck.Api.Features;
+using MicCheck.Api.Features.Usage;
 using MicCheck.Api.Identities;
 using MicCheck.Api.Organizations;
 using MicCheck.Api.Projects;