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");
 

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Repositories/KpiHistoryRepository.cs

@@ -10,7 +10,7 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Repositories;
 /// <c>KpiSample</c> table (M6 "KPI History &amp; Trends"). See the interface for the
 /// contract; this class adds notes on the data-access strategy per method.
 /// </summary>
-public class KpiHistoryRepository : IKpiHistoryRepository
+public sealed class KpiHistoryRepository : IKpiHistoryRepository
 {
     private readonly ScadaBridgeDbContext _context;
 
@@ -29,6 +29,12 @@ public class KpiHistoryRepository : IKpiHistoryRepository
     {
         ArgumentNullException.ThrowIfNull(samples);
 
+        // Avoid a no-op SaveChanges round-trip on quiet sampling ticks.
+        if (samples.Count == 0)
+        {
+            return;
+        }
+
         // Bulk-insert one sampling pass. AddRange + a single SaveChanges keeps the
         // whole batch in one round-trip; the store assigns each row's identity.
         _context.KpiSamples.AddRange(samples);