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>

tests/GmRelay.Bot.Tests/Infrastructure/Database/PlatformIdentityMigrationTests.cs

@@ -117,6 +117,28 @@ public sealed class PlatformIdentityMigrationTests
         Assert.Contains("telegram_id DROP NOT NULL", migration, StringComparison.Ordinal);
     }
 
+    [Fact]
+    public async Task MigrationV024_ShouldDeprecateTelegramColumns()
+    {
+        var migration = await ReadRepositoryFileAsync("src/GmRelay.Bot/Migrations/V024__deprecate_telegram_columns.sql");
+
+        Assert.Contains("UPDATE players", migration, StringComparison.Ordinal);
+        Assert.Contains("UPDATE game_groups", migration, StringComparison.Ordinal);
+        Assert.Contains("DEPRECATED", migration, StringComparison.Ordinal);
+        Assert.Contains("calendar_subscriptions", migration, StringComparison.Ordinal);
+        Assert.Contains("user_platform", migration, StringComparison.Ordinal);
+        Assert.Contains("user_external_id", migration, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public async Task MigrationV025_ShouldBackfillRescheduleProposals()
+    {
+        var migration = await ReadRepositoryFileAsync("src/GmRelay.Bot/Migrations/V025__reschedule_proposals_telegram_external.sql");
+
+        Assert.Contains("reschedule_proposals", migration, StringComparison.Ordinal);
+        Assert.Contains("proposed_by_external_user_id", migration, StringComparison.Ordinal);
+    }
+
     private static async Task<string> ReadRepositoryFileAsync(string relativePath)
     {
         var directory = new DirectoryInfo(AppContext.BaseDirectory);