perf: celery use solo pool (single process) to cut memory

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yaml

@@ -32,7 +32,7 @@ services:
   office-xbot-celery:
     container_name: office-xbot-celery
     image: "${APP_IMAGE}"
-    command: "celery -A app.make_celery worker -B --loglevel INFO --concurrency=2"
+    command: "celery -A app.make_celery worker -B --loglevel INFO --pool=solo"
     environment:
       - APP_ID=${APP_ID}
       - APP_SECRET=${APP_SECRET}