Support setting openai base url in OpenMemory MCP

[FradSer]

Description

Added OPENAI_BASE_URL configuration to config.py and updated OpenAI client instantiation in categorization.py to use it when set. This enables flexible switching of OpenAI endpoints for different environments, supporting environment-based configuration.

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings

[CLAassistant]

All committers have signed the CLA.

[deshraj]

Hey @FradSer, can you please sign the CLA so that we can review this PR?

[mkdir700]

@deshraj Please continue with the next steps, thank you

[FradSer]

@deshraj Just wanted to gently check in on this pull request. Please let me know if there are any questions or changes I can help with.