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.

src/GmRelay.Bot/Migrations/V017__allow_platform_neutral_players.sql

@@ -0,0 +1,9 @@
+-- =============================================================
+-- V017: Allow platform-neutral players
+-- =============================================================
+-- Legacy Telegram identity columns remain for backward compatibility,
+-- but non-Telegram platform users do not have Telegram ids.
+-- =============================================================
+
+ALTER TABLE players
+    ALTER COLUMN telegram_id DROP NOT NULL;