import_viewer: fix counter upsert race + add explicit slot field

Pre-materialize ViewerEventCounters before the mission loop so in-flight
Adds are visible to subsequent iterations sharing the same (EventKey, Period);
add optional ImportMission.Slot field so callers can supply weekly slot identity
(2 or 3) rather than relying on the LotType-derived default of 1.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Admin/ImportViewerRequest.cs

@@ -93,6 +93,9 @@ public class ImportMission
 
     [JsonPropertyName("total_count")]
     public int TotalCount { get; set; }
+
+    [JsonPropertyName("slot")]
+    public int? Slot { get; set; }
 }
 
 public class ImportMissionMeta