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.Web/Services/CalendarSubscriptionService.cs

@@ -12,7 +12,8 @@ public sealed class CalendarSubscriptionService(NpgsqlDataSource dataSource)
     public string GenerateToken() => Guid.NewGuid().ToString("N");
 
     public async Task<string> CreateSubscriptionAsync(
-        long userTelegramId,
+        string userPlatform,
+        string userExternalId,
         Guid? groupId,
         CalendarSubscriptionFilter filter,
         CancellationToken ct = default)
@@ -20,9 +21,9 @@ public sealed class CalendarSubscriptionService(NpgsqlDataSource dataSource)
         var token = GenerateToken();
         await using var connection = await dataSource.OpenConnectionAsync(ct);
         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, groupId, filterType = (int)filter });
+            @"INSERT INTO calendar_subscriptions (id, token, user_platform, user_external_id, group_id, filter_type, created_at, expires_at)
+              VALUES (gen_random_uuid(), @token, @userPlatform, @userExternalId, @groupId, @filterType, now(), NULL)",
+            new { token, userPlatform, userExternalId, groupId, filterType = (int)filter });
         return token;
     }
 
@@ -31,7 +32,7 @@ public sealed class CalendarSubscriptionService(NpgsqlDataSource dataSource)
         await using var connection = await dataSource.OpenConnectionAsync(ct);
 
         var subscription = await connection.QueryFirstOrDefaultAsync<SubscriptionRecord>(
-            @"SELECT id, user_telegram_id as UserTelegramId, group_id as GroupId, filter_type as FilterType
+            @"SELECT id, group_id as GroupId, filter_type as FilterType
               FROM calendar_subscriptions
               WHERE token = @token
                 AND (expires_at IS NULL OR expires_at > now())",
@@ -88,6 +89,6 @@ public sealed class CalendarSubscriptionService(NpgsqlDataSource dataSource)
         .Replace("\n", "\\n")
         .Replace("\r", "");
 
-    private sealed record SubscriptionRecord(Guid Id, long UserTelegramId, Guid? GroupId, int FilterType);
+    private sealed record SubscriptionRecord(Guid Id, Guid? GroupId, int FilterType);
     private sealed record CalendarSessionDto(Guid Id, string Title, DateTime ScheduledAt);
 }