Adds test script for validating PDF preprocessing pipeline:
- Garble rate detection unit tests
- Page number pattern detection unit tests
- Integration tests with demo_docs/edit*.pdf files
- Quality report generation verification
Usage: PYTHONPATH=backend python3 scripts/run_preprocessing_tests.py
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
