Add whitelist feature to restrict GitHub usernames

[verdel]

Description

Introduce a WHITELIST environment variable to restrict which GitHub users the service fetches commit statistics for. Users not in the whitelist are ignored. If WHITELIST is empty or unset, statistics are available for all users.

Type of change

• Bug fix (added a non-breaking change which fixes an issue)
• New feature (added a non-breaking change which adds functionality)
• Updated documentation (updated the readme, templates, or other repo files)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Tested locally with a valid username
• Tested locally with an invalid username
• Ran tests with composer test
• Added or updated test cases to test new features

Checklist:

• I have checked to make sure no other pull requests are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[DenverCoder1]

Looks great! Thanks for contributing!

[verdel]

I’ll fix the documentation comments and double-check the functionality in the Docker container a bit later (in about 2–3 hours). Please don’t merge the PR yet.

[verdel]

I have updated README.md according to the comment in the PR and double-checked that the service works correctly when using the WHITELIST environment variable in the Docker container. Everything works as expected. The PR can now be merged.

Thank you very much for your work and for this widget.