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

TDD cycle for issue #23:
- RED: 7 migration smoke tests (file presence + schema expectations)
- GREEN: V016 migration adding platform identity columns
- GREEN: CreateSessionHandler updated with 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 to legacy and new identity columns
- Add PlatformIdentityMigrationTests (7 smoke tests)
- 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>

compose.yaml

@@ -49,7 +49,7 @@ services:
         crond -f
 
   bot:
-    image: git.codeanddice.ru/toutsu/gmrelay-bot:1.16.0
+    image: git.codeanddice.ru/toutsu/gmrelay-bot:2.0.0
     restart: always
     depends_on:
       db:
@@ -67,7 +67,7 @@ services:
       retries: 3
 
   web:
-    image: git.codeanddice.ru/toutsu/gmrelay-web:1.16.0
+    image: git.codeanddice.ru/toutsu/gmrelay-web:2.0.0
     restart: always
     depends_on:
       db: