Fix max_debt_to_equity filter and add custom criteria parsing

- Fix bug in screen_database_initial where max_debt_to_equity filter was skipped
- Now properly filters stocks by debt-to-equity ratio
- Add comprehensive custom criteria parsing to screener.yaml:
  - Sector mappings (tech, healthcare, financial, etc.)
  - Market cap phrases (under $10B, small cap, mega cap)
  - Quality filters (high margins, low debt, high ROIC)
- Update README with custom screening examples
- Enables natural language queries like "/screen tech stocks with high margins"

Author: pradeepiyer