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

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

@@ -192,6 +192,15 @@ public sealed class KpiHistoryRepository : IKpiHistoryRepository
             .ToListAsync(cancellationToken);
     }
 
+    /// <inheritdoc />
+    public async Task<DateTime?> GetLatestRollupHourAsync(CancellationToken cancellationToken = default)
+    {
+        // Single MAX over the unique IX_KpiRollupHourly_Series population (the rollup
+        // table is small — one row per series-hour). Null when no rollups exist.
+        return await _context.KpiRollupHourly
+            .MaxAsync(r => (DateTime?)r.HourStartUtc, cancellationToken);
+    }
+
     /// <inheritdoc />
     public async Task PurgeRollupsOlderThanAsync(DateTime before, CancellationToken cancellationToken = default)
     {