Remove distutils strtobool dependency

[Mirochill]

Summary

• Replace the distutils.util.strtobool import with a local boolean parser for the same environment string values.
• Keep env-backed PostgreSQL feature flags returning 1 or 0 to preserve current behavior.
• Add focused unit coverage for accepted and invalid boolean strings.

Fixes #14

Validation

• git diff --check HEAD~1..HEAD
• Not run locally: flake8, coverage run