fix(data): align portfolio mutation lock order

src/GmRelay.Shared/Features/Sessions/ListSessions/DeleteSessionHandler.cs

@@ -31,7 +31,7 @@ public sealed class DeleteSessionHandler(
         await using var connection = await dataSource.OpenConnectionAsync(ct);
         await using var transaction = await connection.BeginTransactionAsync(ct);
 
-        // 1. Fetch session and verify group manager.
+        // 1. Lock the session before any linked portfolio card and verify group manager.
         var session = await connection.QuerySingleOrDefaultAsync<DeleteSessionInfoDto>(
             """
             SELECT s.title AS Title,
@@ -49,6 +49,7 @@ public sealed class DeleteSessionHandler(
                    ) AS CanManage
             FROM sessions s
             WHERE s.id = @SessionId
+            FOR UPDATE OF s
             """,
             new { command.SessionId, Platform = command.User.Platform.ToString(), ExternalUserId = command.User.ExternalUserId }, transaction);