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/ListSessions/DeleteSessionHandler.cs

@@ -44,12 +44,13 @@ public sealed class DeleteSessionHandler(
                        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)
         {
@@ -109,18 +110,22 @@ public sealed class DeleteSessionHandler(
                          FROM group_managers gm
                          JOIN players manager_player ON manager_player.id = gm.player_id
                          WHERE gm.group_id = s.group_id
-                           AND manager_player.telegram_id = @TelegramUserId
+                           AND manager_player.platform = 'Telegram'
+                           AND manager_player.external_user_id = @ExternalUserId
                      ) AS CanManage
               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.platform = 'Telegram'
+                AND g.external_group_id = @ExternalChatId
+                AND s.status != @Cancelled
+                AND s.scheduled_at > NOW()
               GROUP BY s.id, s.title, s.scheduled_at, s.status, s.max_players, s.group_id
               ORDER BY s.scheduled_at ASC",
             new
             {
-                ChatId = command.ChatId,
-                command.TelegramUserId,
+                ExternalChatId = command.ChatId.ToString(),
+                ExternalUserId = command.TelegramUserId.ToString(),
                 Cancelled = SessionStatus.Cancelled,
                 Active = ParticipantRegistrationStatus.Active,
                 Waitlisted = ParticipantRegistrationStatus.Waitlisted