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/CreateSession/JoinSessionHandler.cs

@@ -40,30 +40,19 @@ public sealed class JoinSessionHandler(
         {
             // 1. Убеждаемся, что игрок есть в базе
             var platform = command.User.Platform.ToString();
-            var legacyTelegramId = command.User.Platform == PlatformKind.Telegram
-                ? long.Parse(command.User.ExternalUserId, CultureInfo.InvariantCulture)
-                : (long?)null;
-            var legacyTelegramUsername = command.User.Platform == PlatformKind.Telegram
-                ? command.User.ExternalUsername
-                : null;
 
             var playerId = await connection.ExecuteScalarAsync<Guid>(
-                @"INSERT INTO players (telegram_id, display_name, telegram_username, platform, external_user_id, external_username)
-                  VALUES (@LegacyTelegramId, @Name, @LegacyTelegramUsername, @Platform, @ExternalUserId, @ExternalUsername)
+                @"INSERT INTO players (display_name, platform, external_user_id, external_username)
+                  VALUES (@Name, @Platform, @ExternalUserId, @ExternalUsername)
                   ON CONFLICT (platform, external_user_id)
                   WHERE platform IS NOT NULL AND external_user_id IS NOT NULL
                   DO UPDATE
                   SET display_name = EXCLUDED.display_name,
-                      telegram_username = COALESCE(EXCLUDED.telegram_username, players.telegram_username),
-                      platform = EXCLUDED.platform,
-                      external_user_id = EXCLUDED.external_user_id,
                       external_username = EXCLUDED.external_username
                   RETURNING id;",
                 new
                 {
-                    LegacyTelegramId = legacyTelegramId,
                     Name = command.User.DisplayName,
-                    LegacyTelegramUsername = legacyTelegramUsername,
                     Platform = platform,
                     command.User.ExternalUserId,
                     command.User.ExternalUsername
@@ -155,7 +144,7 @@ public sealed class JoinSessionHandler(
             var batchParticipants = 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 sp.player_id = p.id

src/GmRelay.Shared/Features/Sessions/CreateSession/LeaveSessionHandler.cs

@@ -173,7 +173,7 @@ public sealed class LeaveSessionHandler(
                 """
                 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 sp.player_id = p.id