[jschm42]

Summary

Migrated beeboard2 backend dependency management from requirements.txt to Poetry (PEP 621 compliant).

Changes

• Configured backend/pyproject.toml and generated backend/poetry.lock.
• Added backend/poetry.toml with virtualenvs.in-project = true.
• Updated docker/Dockerfile.backend and docker/Dockerfile.unified to use Poetry.
• Updated setup scripts (scripts/setup.sh, scripts/setup.ps1).
• Updated GitHub Actions CI workflow (.github/workflows/quality.yml).
• Removed obsolete backend/requirements.txt.
• Verified test suite and linting (pytest, ruff).