fix(shared): convert GameSystem to string in SQL, guard rollback after commit

src/GmRelay.Shared/Features/Sessions/CreateSession/CreateSessionHandler.cs

@@ -16,6 +16,7 @@ public sealed class CreateSessionHandler(
         await using var connection = await dataSource.OpenConnectionAsync(ct);
         await using var transaction = await connection.BeginTransactionAsync(ct);
 
+        var transactionCommitted = false;
         try
         {
             var platform = command.User.Platform.ToString();
@@ -130,7 +131,7 @@ public sealed class CreateSessionHandler(
                         ScheduledAt = scheduledAt,
                         Status = SessionStatus.Planned,
                         MaxPlayers = command.MaxPlayers,
-                        System = command.System,
+                        System = command.System?.ToString(),
                         command.Description,
                         command.Format,
                         DurationMinutes = command.DurationMinutes,
@@ -142,6 +143,7 @@ public sealed class CreateSessionHandler(
             }
 
             await transaction.CommitAsync(ct);
+            transactionCommitted = true;
 
             var view = SessionBatchViewBuilder.Build(command.Title, sessions, Array.Empty<ParticipantBatchDto>());
 
@@ -155,7 +157,10 @@ public sealed class CreateSessionHandler(
         }
         catch
         {
-            await transaction.RollbackAsync(ct);
+            if (!transactionCommitted)
+            {
+                await transaction.RollbackAsync(ct);
+            }
             throw;
         }
     }