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.DiscordBot/Features/Sessions/DiscordListSessionsHandler.cs

@@ -74,7 +74,7 @@ public sealed class DiscordListSessionsHandler(
         var participants = await connection.QueryAsync<ParticipantBatchDto>(
             @"SELECT sp.session_id as SessionId,
                      p.display_name as DisplayName,
-                     COALESCE(p.external_username, p.telegram_username) as TelegramUsername,
+                     p.external_username as TelegramUsername,
                      sp.registration_status as RegistrationStatus
               FROM session_participants sp
               JOIN players p ON p.id = sp.player_id

src/GmRelay.DiscordBot/Features/Sessions/DiscordRescheduleVotingDeadlineService.cs

@@ -152,7 +152,7 @@ public sealed class DiscordRescheduleVotingDeadlineService(
                 """
                 SELECT sp.session_id AS SessionId,
                        p.display_name AS DisplayName,
-                       COALESCE(p.external_username, p.telegram_username) AS TelegramUsername,
+                       p.external_username AS TelegramUsername,
                        sp.registration_status AS RegistrationStatus
                 FROM session_participants sp
                 JOIN players p ON p.id = sp.player_id