Add environment variable CELERY_BEAT_SCHEDULE_FILENAME to support read-only rootfs environments

[potix2]

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

When running Dify on AWS EKS, the container root filesystem is read-only.
Celery Beat tries to write its schedule file to the default path and fails with a PermissionError.
By allowing the schedule file path to be customized, Celery Beat can run normally in these environments.

2. Additional context or comments

I have tested the custom schedule file path on AWS EKS using /tmp/celerybeat-schedule.db, and Celery Beat works correctly. This change would make Dify more compatible with read-only rootfs environments.

3. Can you help us with this feature?

• I am interested in contributing to this feature.