feat: scheduled tasks

[owengo]

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Summary

Adds a per‑user scheduling system that can run an agent automatically on a cron schedule, create a new conversation on success, and record failures. Users can create multiple schedules per agent and edit/enable/disable them from the new Scheduled Tasks panel. The UI includes a zero‑knowledge cron builder (minute/hour/day/month dropdowns) with a human‑readable preview plus an Advanced mode for direct cron input; timezone is required. On the backend, a scheduler service executes tasks with queueing/concurrency limits, max runtime enforcement, and failure logging; it uses the latest agent version at run time and supports agent variables (e.g., {{current_date}}). Admins can set a global minimum interval and max concurrent runs in librechat.yaml to prevent spikes.

Change Type

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Translation update

Testing

This is a draft, all code is generated by codex.
I will add a test suite and documentation if it has some chance to be accepted.

Test Configuration:

Checklist

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• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules.
• A pull request for updating the documentation has been submitted.