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/Confirmation/SendConfirmation/SendConfirmationHandler.cs

@@ -45,10 +45,10 @@ public sealed class SendConfirmationHandler(
                    s.title,
                    s.scheduled_at AS ScheduledAt,
                    s.group_id AS GroupId,
-                   COALESCE(g.platform, 'Telegram') AS Platform,
-                   COALESCE(g.external_group_id, g.telegram_chat_id::TEXT) AS ExternalGroupId,
+                   g.platform AS Platform,
+                   g.external_group_id AS ExternalGroupId,
                    g.name AS DisplayName,
-                   COALESCE(g.external_channel_id, g.telegram_chat_id::TEXT) AS ExternalChannelId,
+                   g.external_channel_id AS ExternalChannelId,
                    s.thread_id AS ThreadId,
                    s.notification_mode AS NotificationMode
             FROM sessions s
@@ -65,10 +65,10 @@ public sealed class SendConfirmationHandler(
 
         var participants = (await connection.QueryAsync<ConfirmationParticipantRow>(
             """
-            SELECT COALESCE(p.platform, 'Telegram') AS Platform,
-                   COALESCE(p.external_user_id, p.telegram_id::TEXT) AS ExternalUserId,
+            SELECT p.platform AS Platform,
+                   p.external_user_id AS ExternalUserId,
                    p.display_name AS DisplayName,
-                   COALESCE(p.external_username, p.telegram_username) AS ExternalUsername,
+                   p.external_username AS ExternalUsername,
                    sp.rsvp_status AS RsvpStatus,
                    sp.registration_status AS RegistrationStatus,
                    sp.is_gm AS IsGm