refactor(kpi): K2/K6/K7 review fixups — empty-batch guard + sealed repo + uniform TryAddEnumerable + KPI-age doc fidelity + coverage

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Configurations/KpiSampleEntityTypeConfiguration.cs

@@ -45,6 +45,8 @@ public class KpiSampleEntityTypeConfiguration : IEntityTypeConfiguration<KpiSamp
 
         // Series index — backs the bucketed query path (filter one series, scan in
         // capture order). Names locked for migration discoverability.
+        // Global-scope rows (ScopeKey IS NULL) are included in the index and are
+        // seeked via the IS NULL predicate, so the index covers all scope levels.
         builder.HasIndex(s => new { s.Source, s.Metric, s.Scope, s.ScopeKey, s.CapturedAtUtc })
             .HasDatabaseName("IX_KpiSample_Series");