feat: add configurable celery beat schedule filename

[potix2]

Fixes #28530

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Summary

This change adds support for customizing the Celery Beat schedule file path via the CELERY_BEAT_SCHEDULE_FILENAME environment variable. This is useful in Kubernetes environments where the root filesystem is read-only, as Celery Beat cannot create its default schedule file. By allowing the schedule file location to be configured (e.g., /tmp/celerybeat-schedule.db), deployments can avoid write-permission issues without modifying the application code.

No additional dependencies are introduced by this change.

You can verify the correctness of this change in the official Celery documentation. Celery Beat supports customizing the schedule database path using the --schedule (-s) option, as described here:

https://docs.celeryq.dev/en/stable/reference/cli.html#beat

According to the documentation, the --schedule option allows specifying an alternative path for the schedule file instead of the default celerybeat-schedule. This confirms that providing a custom schedule file path (e.g., via the CELERY_BEAT_SCHEDULE_FILENAME environment variable) is an intended and supported behavior in Celery.

Screenshots

Before	After

Checklist

• [x] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist[bot]]

Summary of Changes

Hello @potix2, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the application's deployment flexibility by allowing the Celery Beat schedule file path to be configured via an environment variable. This is particularly beneficial for containerized deployments, such as those in Kubernetes, where default file locations might not be writable, ensuring the proper functioning of scheduled tasks without requiring modifications to the application's core code.

Highlights

• Configurable Celery Beat Schedule Filename: Introduced support for customizing the Celery Beat schedule file path using the CELERY_BEAT_SCHEDULE_FILENAME environment variable.
• Kubernetes Compatibility: This change specifically addresses issues in Kubernetes environments where the root filesystem is often read-only, preventing Celery Beat from creating its default schedule file.
• Leverages Celery's --schedule option: The implementation utilizes Celery's built-in --schedule command-line option, allowing the schedule file location to be specified dynamically.

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[asukaminato0721]

/gemini review