refactor: remove unused EXTERNAL_ADMIN_TOKEN functionality

- Delete app/services/external_admin_service.py entirely
- Remove EXTERNAL_ADMIN_TOKEN and EXTERNAL_ADMIN_TOKEN_BOT_ID from config
- Remove build_external_admin_token, get_external_admin_token, get_external_admin_bot_id methods
- Remove unused hashlib/hmac imports from config.py
- Remove from system_settings_service: READ_ONLY_KEYS, PLAIN_TEXT_KEYS,
  category title, category description, prefix mapping, documentation metadata
- Remove from bot_configuration.py category group
- Remove from main.py startup sequence (ensure_external_admin_token call)
- Remove from .env.example
- Remove from docs/project_structure_reference.md

main.py

@@ -22,7 +22,6 @@ from app.services.ban_notification_service import ban_notification_service
 from app.services.broadcast_service import broadcast_service
 from app.services.contest_rotation_service import contest_rotation_service
 from app.services.daily_subscription_service import daily_subscription_service
-from app.services.external_admin_service import ensure_external_admin_token
 from app.services.log_rotation_service import log_rotation_service
 from app.services.maintenance_service import maintenance_service
 from app.services.monitoring_service import monitoring_service
@@ -515,24 +514,6 @@ async def main():
             else:
                 stage.skip('NaloGO отключен настройками')
 
-        async with timeline.stage(
-            'Внешняя админка',
-            '🛡️',
-            success_message='Токен внешней админки готов',
-        ) as stage:
-            try:
-                token = await ensure_external_admin_token(
-                    bot_user.username,
-                    bot_user.id,
-                )
-                if token:
-                    stage.log('Токен синхронизирован')
-                else:
-                    stage.warning('Не удалось получить токен внешней админки')
-            except Exception as error:  # pragma: no cover - защитный блок
-                stage.warning(f'Ошибка подготовки внешней админки: {error}')
-                logger.error('❌ Ошибка подготовки внешней админки', error=error)
-
         bot_run_mode = settings.get_bot_run_mode()
         polling_enabled = bot_run_mode == 'polling'
         telegram_webhook_enabled = bot_run_mode == 'webhook'