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>

.gitea/workflows/deploy.yml

@@ -6,7 +6,7 @@ on:
       - main 
 
 env:
-  VERSION: 1.16.0
+  VERSION: 2.0.0
 
 jobs:
   # ЧАСТЬ 1: Собираем образы и кладем в Gitea (чтобы делиться с ребятами)