fix: support regex in global filters

[Iams4kura]

Summary

• Fixes: A title containing spam still passes a documented /spam|clickbait/ global filter.
• Root cause: Global filters are stored as raw strings and matched by substring, so slash-delimited regular expressions are treated literally.

Regression evidence

• Before: PYTHONDONTWRITEBYTECODE=1 uv run --frozen python -m unittest discover -s tests -p 'test_frequency.py' -v exited 1

• After: PYTHONDONTWRITEBYTECODE=1 uv run --frozen python -m unittest discover -s tests -p 'test_frequency.py' -v exited 0

Verification

• PYTHONDONTWRITEBYTECODE=1 uv run --frozen python -m unittest discover -s tests -v
• uvx ruff check tests/test_frequency.py
• uv build --no-sources

Scope

• 2 files changed, +49 / -5 lines

Authorship

This fix was developed with AI assistance and was validated with the regression and project checks listed above.