review(bp): doc fixes + sales_period_info non-nullable + drop checked cast + TODO

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

SVSim.Database/Repositories/BattlePass/IViewerBattlePassRepository.cs

@@ -6,8 +6,10 @@ namespace SVSim.Database.Repositories.BattlePass;
 public interface IViewerBattlePassRepository
 {
     /// <summary>
-    /// Get-or-create progress row for (viewer, season). New rows are added to the change-tracker
-    /// but NOT saved — caller batches with other mutations.
+    /// Get-or-create progress row for (viewer, season). New rows are saved IMMEDIATELY (with
+    /// DbUpdateException catch-and-retry to handle the concurrent-first-visit race against
+    /// the (ViewerId, SeasonId) unique index). Existing rows are returned tracked so callers
+    /// can mutate them and batch the save with other changes.
     /// </summary>
     Task<ViewerBattlePassProgressEntry> GetOrCreateProgressAsync(long viewerId, int seasonId, CancellationToken ct);