perf(sql): sweep/KPI covering indexes + sliced notification terminal purge

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardSchema.cs

@@ -94,6 +94,23 @@ public static class StoreAndForwardSchema
                 "CREATE INDEX IF NOT EXISTS idx_sf_messages_status_due ON sf_messages(status, last_attempt_at_ms)";
             dueIndex.ExecuteNonQuery();
         }
+
+        // Covering index for GetMessagesForRetryAsync's ORDER BY created_at ASC.
+        // idx_sf_messages_status_due (above) matches the status filter and the
+        // last_attempt_at_ms half of the due predicate, but its column order does
+        // NOT match the query's "ORDER BY created_at ASC", so SQLite still needs a
+        // sort/scan step to satisfy that ordering on a large due-sweep. This index's
+        // leading (status, created_at) column order lets the planner walk matching
+        // rows already in created_at order and stop at LIMIT without touching
+        // non-pending rows or re-sorting (arch-review WP1.4). idx_sf_messages_status_due
+        // is left in place — it still backs status+due-time lookups that don't order by
+        // created_at.
+        using (var orderIndex = connection.CreateCommand())
+        {
+            orderIndex.CommandText =
+                "CREATE INDEX IF NOT EXISTS idx_sf_messages_status_created ON sf_messages(status, created_at)";
+            orderIndex.ExecuteNonQuery();
+        }
     }
 
     /// <summary>