feat(kpi): GetLatestRollupHourAsync watermark seam for the backfill fast-path (plan R2-04 T2)

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Repositories/IKpiHistoryRepository.cs

@@ -109,6 +109,16 @@ public interface IKpiHistoryRepository
         string source, string metric, string scope, string? scopeKey,
         DateTime fromUtc, DateTime toUtc, CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Returns the newest <c>KpiRollupHourly.HourStartUtc</c> across all series, or
+    /// <c>null</c> when no rollups exist. The recorder's one-shot backfill consults
+    /// this watermark to skip (or shrink to) the un-rolled tail instead of re-folding
+    /// the whole raw-retention window on every failover (arch-review 04 round 2, R1).
+    /// </summary>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>A task resolving to the newest folded hour start, or <c>null</c> when empty.</returns>
+    Task<DateTime?> GetLatestRollupHourAsync(CancellationToken cancellationToken = default);
+
     /// <summary>
     /// Bulk-deletes hourly rollup rows whose
     /// <see cref="ZB.MOM.WW.ScadaBridge.Commons.Entities.Kpi.KpiRollupHourly.HourStartUtc"/> is