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

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)
     {