fix celery config (#22566)

Signed-off-by: kenwoodjw <blackxin55+@gmail.com>

api/.env.example

@@ -54,7 +54,7 @@ REDIS_CLUSTERS_PASSWORD=
 
 # celery configuration
 CELERY_BROKER_URL=redis://:difyai123456@localhost:${REDIS_PORT}/1
-
+CELERY_BACKEND=redis
 # PostgreSQL database configuration
 DB_USERNAME=postgres
 DB_PASSWORD=difyai123456

api/configs/middleware/__init__.py

@@ -211,7 +211,7 @@ class DatabaseConfig(BaseSettings):
 class CeleryConfig(DatabaseConfig):
     CELERY_BACKEND: str = Field(
         description="Backend for Celery task results. Options: 'database', 'redis'.",
-        default="database",
+        default="redis",
     )
 
     CELERY_BROKER_URL: Optional[str] = Field(