fix(kpi): classify the failover fold-race pass loss as a self-healing Information event (plan R2-04 T9)

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

@@ -1,4 +1,6 @@
 using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Kpi;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Kpi;
@@ -13,14 +15,22 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Repositories;
 public sealed class KpiHistoryRepository : IKpiHistoryRepository
 {
     private readonly ScadaBridgeDbContext _context;
+    private readonly ILogger<KpiHistoryRepository> _logger;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="KpiHistoryRepository"/> class.
     /// </summary>
     /// <param name="context">The EF Core database context.</param>
-    public KpiHistoryRepository(ScadaBridgeDbContext context)
+    /// <param name="logger">
+    /// Optional logger; defaults to <see cref="NullLogger{T}"/> (mirrors
+    /// <c>SiteCallAuditRepository</c> — MS.DI resolves <see cref="ILogger{T}"/> automatically,
+    /// so no registration churn). Used to classify the self-healing failover fold-race.
+    /// </param>
+    public KpiHistoryRepository(
+        ScadaBridgeDbContext context, ILogger<KpiHistoryRepository>? logger = null)
     {
         _context = context ?? throw new ArgumentNullException(nameof(context));
+        _logger = logger ?? NullLogger<KpiHistoryRepository>.Instance;
     }
 
     /// <inheritdoc />
@@ -170,7 +180,22 @@ public sealed class KpiHistoryRepository : IKpiHistoryRepository
             }
         }
 
-        await _context.SaveChangesAsync(cancellationToken);
+        try
+        {
+            await _context.SaveChangesAsync(cancellationToken);
+        }
+        catch (DbUpdateException ex)
+        {
+            // Failover-overlap race: the old singleton incarnation's in-flight fold and the
+            // new node's first fold can both Add the same (series, hour) row; the unique
+            // IX_KpiRollupHourly_Series then faults the loser's ENTIRE SaveChanges — the
+            // failure grain is the PASS, not the row (arch-review 04 round 2, R5). The fold
+            // only ever writes KpiRollupHourly rows, so any save fault here is a lost upsert
+            // race or a transient the next idempotent re-fold repairs identically; classify
+            // at Information instead of surfacing a scary error for a self-healing no-op.
+            _logger.LogInformation(ex,
+                "KPI rollup fold lost a failover-overlap upsert race; pass discarded, next fold self-heals.");
+        }
     }
 
     /// <inheritdoc />