fix(data): serialize portfolio mutations before rows

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

@@ -31,7 +31,12 @@ public sealed class DeleteSessionHandler(
         await using var connection = await dataSource.OpenConnectionAsync(ct);
         await using var transaction = await connection.BeginTransactionAsync(ct);
 
-        // 1. Lock the session before any linked portfolio card and verify group manager.
+        // 1. Use the database mutation order before locking the session or linked portfolio cards.
+        await connection.ExecuteAsync(
+            "SELECT pg_advisory_xact_lock(20260530, 108)",
+            transaction: transaction);
+
+        // 2. Lock the session before any linked portfolio card and verify group manager.
         var session = await connection.QuerySingleOrDefaultAsync<DeleteSessionInfoDto>(
             """
             SELECT s.title AS Title,
@@ -63,7 +68,7 @@ public sealed class DeleteSessionHandler(
             return new DeleteSessionResult(false, "Только owner или co-GM может удалять сессию.", null, null, null, false, 0);
         }
 
-        // 2. Unpublish a linked portfolio card before its required session link cascades away.
+        // 3. Unpublish a linked portfolio card before its required session link cascades away.
         await connection.ExecuteAsync(
             """
             UPDATE portfolio_games pg
@@ -77,7 +82,7 @@ public sealed class DeleteSessionHandler(
             new { command.SessionId },
             transaction);
 
-        // 3. Delete session
+        // 4. Delete session
         await connection.ExecuteAsync("DELETE FROM sessions WHERE id = @Id", new { Id = command.SessionId }, transaction);
 
         var remainingInTopic = session.ThreadId.HasValue