refactor: deprecate root PII scrubber

[huazhuang80-star]

Closes #112.

Summary

• remove the legacy regex-only scrub_pii implementation from the root scrubber.py
• leave the root module as a deprecation notice pointing to app/ai-service/services/pii_scrubber.py
• update the root PII scrubber test to assert the legacy function is gone and the canonical PIIScrubberService.anonymize implementation remains present

Testing

• python -m pytest tests\test_pii_scrubber.py -q