refactor: завершить platform migration и удалить deprecated telegram_* scaffolding

- Добавлены миграции V024 (backfill + deprecation comments + calendar_subscriptions platform identity) и V025 (backfill proposed_by_external_user_id)
- Все Bot handlers переведены с telegram_id/chat_id на platform + external_*
- Shared handlers очищены от COALESCE fallback с telegram_* колонками
- DiscordBot очищен от COALESCE fallback
- Web SessionService и CalendarSubscriptionService переведены на external_*
- HandleRsvpHandler: убран legacy UNION с gm_telegram_id, теперь только group_managers
- RescheduleVotingFinalizer: переведен на external_username/external_user_id
- Tests: добавлены asserts для V024/V025
- Версия обновлена до 3.1.0

Bump version → 3.1.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Bot/Features/Sessions/CreateSession/CancelSessionHandler.cs

@@ -42,12 +42,13 @@ public sealed class CancelSessionHandler(
                        FROM group_managers gm
                        JOIN players p ON p.id = gm.player_id
                        WHERE gm.group_id = s.group_id
-                         AND p.telegram_id = @TelegramUserId
+                         AND p.platform = 'Telegram'
+                         AND p.external_user_id = @ExternalUserId
                    ) AS CanManage
             FROM sessions s
             WHERE s.id = @SessionId
             """,
-            new { command.SessionId, command.TelegramUserId }, transaction);
+            new { command.SessionId, ExternalUserId = command.TelegramUserId.ToString() }, transaction);
 
         if (session == null)
         {
@@ -89,7 +90,7 @@ public sealed class CancelSessionHandler(
 
         var directRecipients = (await connection.QueryAsync<DirectNotificationRecipient>(
             """
-            SELECT p.telegram_id AS TelegramId,
+            SELECT p.external_user_id::BIGINT AS TelegramId,
                    p.display_name AS DisplayName
             FROM session_participants sp
             JOIN players p ON sp.player_id = p.id