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.Shared/Features/Sessions/RescheduleSession/RescheduleVotingFinalizer.cs

@@ -78,8 +78,8 @@ public sealed class RescheduleVotingFinalizer(
             """
             SELECT p.id AS PlayerId,
                    p.display_name AS DisplayName,
-                   p.telegram_username AS TelegramUsername,
-                   p.telegram_id AS TelegramId
+                   p.external_username AS TelegramUsername,
+                   p.external_user_id::BIGINT AS TelegramId
             FROM session_participants sp
             JOIN players p ON p.id = sp.player_id
             WHERE sp.session_id = @SessionId
@@ -107,7 +107,7 @@ public sealed class RescheduleVotingFinalizer(
             SELECT rov.option_id AS OptionId,
                    p.id AS PlayerId,
                    p.display_name AS DisplayName,
-                   p.telegram_username AS TelegramUsername
+                   p.external_username AS TelegramUsername
             FROM reschedule_option_votes rov
             JOIN players p ON p.id = rov.player_id
             WHERE rov.proposal_id = @ProposalId