fix(staging): add APP_KEY_FILE environment variable to scheduler and queue-worker

The scheduler and queue-worker containers were crashing with
RequiredEnvironmentVariableException because the APP_KEY_FILE
environment variable was not set, even though the app_key secret
was mounted. The Framework's Environment class needs the *_FILE
pattern to read Docker Secrets.

docker-compose.staging.yml

@@ -397,6 +397,7 @@ services:
       - DB_PASSWORD=${DB_PASSWORD}
       # Use Docker Secrets via *_FILE pattern (Framework supports this automatically)
       - DB_PASSWORD_FILE=/run/secrets/db_user_password
+      - APP_KEY_FILE=/run/secrets/app_key
       # Redis
       - REDIS_HOST=redis
       - REDIS_PORT=6379
@@ -452,6 +453,7 @@ services:
       - DB_PASSWORD=${DB_PASSWORD}
       # Use Docker Secrets via *_FILE pattern (Framework supports this automatically)
       - DB_PASSWORD_FILE=/run/secrets/db_user_password
+      - APP_KEY_FILE=/run/secrets/app_key
       # Redis
       - REDIS_HOST=redis
       - REDIS_PORT=6379