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/ExportCalendar/ExportCalendarHandler.cs

@@ -24,11 +24,12 @@ public sealed class ExportCalendarHandler(
             @"SELECT s.id as Id, s.title as Title, s.scheduled_at as ScheduledAt"
             + " FROM sessions s"
             + " JOIN game_groups g ON s.group_id = g.id"
-            + " WHERE g.telegram_chat_id = @ChatId"
+            + " WHERE g.platform = 'Telegram'"
+            + " AND g.external_group_id = @ExternalChatId"
             + " AND s.status = @Planned"
             + " AND s.scheduled_at > NOW()"
             + " ORDER BY s.scheduled_at ASC",
-            new { ChatId = message.Chat.Id, Planned = SessionStatus.Planned });
+            new { ExternalChatId = message.Chat.Id.ToString(), Planned = SessionStatus.Planned });
 
         var sessionsList = sessions.ToList();
 
@@ -75,13 +76,13 @@ public sealed class ExportCalendarHandler(
             {
                 var token = Guid.NewGuid().ToString("N");
                 var groupId = await connection.QueryFirstOrDefaultAsync<Guid?>(
-                    @"SELECT id FROM game_groups WHERE telegram_chat_id = @ChatId",
-                    new { ChatId = message.Chat.Id });
+                    @"SELECT id FROM game_groups WHERE platform = 'Telegram' AND external_group_id = @ExternalChatId",
+                    new { ExternalChatId = message.Chat.Id.ToString() });
 
                 await connection.ExecuteAsync(
-                    @"INSERT INTO calendar_subscriptions (id, token, user_telegram_id, group_id, filter_type, created_at, expires_at)
-                      VALUES (gen_random_uuid(), @token, @userTelegramId, @groupId, @filterType, now(), NULL)",
-                    new { token, userTelegramId = senderId.Value, groupId, filterType = (int)CalendarSubscriptionFilter.SpecificGroup });
+                    @"INSERT INTO calendar_subscriptions (id, token, user_platform, user_external_id, group_id, filter_type, created_at, expires_at)
+                      VALUES (gen_random_uuid(), @token, 'Telegram', @userExternalId, @groupId, @filterType, now(), NULL)",
+                    new { token, userExternalId = senderId.Value.ToString(), groupId, filterType = (int)CalendarSubscriptionFilter.SpecificGroup });
 
                 subscriptionUrl = $"{baseUrl.TrimEnd('/')}/calendar/{token}.ics";
             }