Fix subprocess issue on Windows and improve development experience

[kroonen]

Pull Request Checklist

• Description: Briefly describe the changes in this pull request.
• Changelog: Ensure a changelog entry following the format of Keep a Changelog is added at the bottom of the PR description.
• Documentation: Have you updated relevant documentation?
• Dependencies: Are there any new dependencies? Have you updated the dependency versions in the documentation?

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Description

This pull request addresses several issues and makes improvements to the Open WebUI backend:

1. Fixes a syntax warning in the main.py file caused by invalid escape sequences in the ASCII art. The fix involves adding the r prefix before the triple quotes to treat the string as a raw string, eliminating the syntax warning.

2. Resolves the subprocess issue on Windows by modifying the run_background_process function in apps\litellm\main.py. The function now uses subprocess.Popen instead of the asyncio subprocess functions to create a subprocess, ensuring compatibility with Windows.

3. Updates the dev_windows.bat script to include the necessary changes for handling the WEBUI_SECRET_KEY and adding the --reload flag for development purposes.

These changes improve the stability and development experience of the Open WebUI backend on Windows systems.

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Changelog Entry

Fixed

• Fixed a syntax warning in main.py caused by invalid escape sequences in the ASCII art.
• Resolved the subprocess issue on Windows by modifying the run_background_process function in apps\litellm\main.py to use subprocess.Popen.

Changed

• Updated the dev_windows.bat script to handle the WEBUI_SECRET_KEY and add the --reload flag for development purposes.

Documentation

• Updated the development documentation to include instructions for running the backend on Windows using the modified dev_windows.bat script.

Dependencies

• No new dependencies were added.
• No changes to the dependency versions were made.