refactor: make session join leave platform-neutral

Convert join/leave interaction commands to PlatformUser, PlatformGroup, and PlatformMessageRef. Persist and look up participants by platform identity while keeping Telegram callbacks intact. Add V017 migration and TDD coverage. Bump version to 2.1.1.

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

@@ -99,6 +99,15 @@ public sealed class PlatformIdentityMigrationTests
         Assert.Contains("external_username", statsMigration, StringComparison.Ordinal);
     }
 
+    [Fact]
+    public async Task MigrationV017_ShouldAllowPlayersWithoutLegacyTelegramId()
+    {
+        var migration = await ReadRepositoryFileAsync("src/GmRelay.Bot/Migrations/V017__allow_platform_neutral_players.sql");
+
+        Assert.Contains("ALTER TABLE players", migration, StringComparison.Ordinal);
+        Assert.Contains("telegram_id DROP NOT NULL", migration, StringComparison.Ordinal);
+    }
+
     private static async Task<string> ReadRepositoryFileAsync(string relativePath)
     {
         var directory = new DirectoryInfo(AppContext.BaseDirectory);