review(bp): move SaveChanges into repo with race protection; JST constant

GetOrCreateProgressAsync now persists the new row itself and catches
DbUpdateException on unique-constraint violations — concurrent /info
calls no longer throw 500s. BattlePassService no longer calls
SaveChangesAsync after the get-or-create. FormatWireDate uses a named
JstOffset constant instead of an inline TimeSpan.FromHours(9).

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

SVSim.Database/Repositories/BattlePass/ViewerBattlePassRepository.cs

@@ -26,7 +26,18 @@ public sealed class ViewerBattlePassRepository : IViewerBattlePassRepository
             WeeklyPeriodStart = null,
         };
         _db.ViewerBattlePassProgress.Add(entry);
-        return entry;
+        try
+        {
+            await _db.SaveChangesAsync(ct);
+            return entry;
+        }
+        catch (DbUpdateException)
+        {
+            // Concurrent /info call won the race; re-read the row the other thread persisted.
+            _db.Entry(entry).State = EntityState.Detached;
+            return await _db.ViewerBattlePassProgress
+                .FirstAsync(p => p.ViewerId == viewerId && p.SeasonId == seasonId, ct);
+        }
     }
 
     public Task<List<ViewerBattlePassClaimEntry>> GetClaimsAsync(long viewerId, int seasonId, CancellationToken ct) =>