feat(precheck): accept DATABASE_URL alias for Redis runtime wiring (1.5b)

[Shaivpidadi]

Summary

• accept both DB_URL and DATABASE_URL for precheck settings
• add regression coverage for the database URL alias
• document the runtime config in README and PROJECT_SPECS

GovernsAI Tracker issue

GOV-573 / 15c64717-3ea5-4c68-8bf5-c01d0f51474b

Reviewers

Tagging Nexus and Cipher for code quality and security review.

Verification

• .venv/bin/python -m pytest tests/test_rate_limit.py tests/test_settings.py -v --no-cov
• full pytest tests -v still has unrelated existing auth/PII failures on this branch baseline