refactor: unify session status model

Fixes #5

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

@@ -80,10 +80,10 @@ public sealed class DeleteSessionHandler(
               FROM sessions s
               JOIN game_groups g ON s.group_id = g.id
               LEFT JOIN session_participants sp ON s.id = sp.session_id
-              WHERE g.telegram_chat_id = @ChatId AND s.status != 'Cancelled' AND s.scheduled_at > NOW()
+              WHERE g.telegram_chat_id = @ChatId AND s.status != @Cancelled AND s.scheduled_at > NOW()
               GROUP BY s.id, s.title, s.scheduled_at, s.status, g.gm_telegram_id
               ORDER BY s.scheduled_at ASC",
-            new { ChatId = command.ChatId });
+            new { ChatId = command.ChatId, Cancelled = SessionStatus.Cancelled });
 
         var sessionsList = sessions.ToList();