refactor: remove legacy PII scrubber

[huazhuang80-star]

Summary

• remove the root-level legacy scrubber.py implementation
• point the existing PII regression test at �pp/ai-service/services/pii_scrubber.py
• update legacy fixtures to match the canonical service token names

Fixes #112

Testing

• git diff --check
• Not run: python -m pytest tests/test_pii_scrubber.py because the local Python launcher exits without a usable interpreter in this shell
• Not run: bundled Python lacks project test dependencies (pytest, then prometheus_client when checking the service import directly)