chore: add platform identity and platform_messages for multi-platform support (#23)

TDD cycle for issue #23:
- RED: 9 migration smoke tests (file presence + schema expectations)
- GREEN: V016 migration adding platform identity columns
- GREEN: CreateSessionHandler, JoinSessionHandler, Web SessionService updated
  with dual-write to legacy and new identity columns + COALESCE fallbacks
- GREEN: get_group_attendance_stats recreated for external_username
- Bump version to 2.0.0

Changes:
- V016__add_platform_identity.sql:
  - players: platform, external_user_id, external_username
  - game_groups: platform, external_group_id, external_channel_id
  - platform_messages table with cross-platform message tracking
  - Backfill all existing Telegram data into new columns
  - Recreate get_group_attendance_stats with COALESCE fallback
- V012__add_attendance_stats.sql: use COALESCE(external_username, telegram_username)
- CreateSessionHandler: dual-write + COALESCE fallbacks in SELECTs
- JoinSessionHandler: dual-write to new identity columns
- Web SessionService: dual-write to new identity columns
- PlatformIdentityMigrationTests (9 smoke tests covering all handlers)
- Version synced: Directory.Build.props, compose.yaml, deploy.yml, NavMenu.razor → 2.0.0

Legacy telegram_* columns preserved for backward compatibility.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Web/Services/SessionService.cs

@@ -242,11 +242,14 @@ public sealed class SessionService(
 
         await conn.ExecuteAsync(
             """
-            INSERT INTO players (telegram_id, display_name, telegram_username)
-            VALUES (@TelegramId, @DisplayName, @TelegramUsername)
+            INSERT INTO players (telegram_id, display_name, telegram_username, platform, external_user_id, external_username)
+            VALUES (@TelegramId, @DisplayName, @TelegramUsername, 'Telegram', @TelegramId::TEXT, @TelegramUsername)
             ON CONFLICT (telegram_id) DO UPDATE
             SET display_name = EXCLUDED.display_name,
-                telegram_username = EXCLUDED.telegram_username
+                telegram_username = EXCLUDED.telegram_username,
+                platform = COALESCE(players.platform, 'Telegram'),
+                external_user_id = COALESCE(players.external_user_id, EXCLUDED.telegram_id::TEXT),
+                external_username = COALESCE(players.external_username, EXCLUDED.telegram_username)
             """,
             new
             {