chore: migrate to uv for Python dependency management

[Joe-Heffer-Shef]

Proposal

Migrate from requirements.txt / requirements-dev.txt to uv with a pyproject.toml, enabling uv sync for reproducible installs.

Motivation

• uv sync generates a lockfile (uv.lock) for fully reproducible environments
• pyproject.toml is the modern Python standard for declaring project metadata and dependencies
• uv is significantly faster than pip for installs

Work required

1. Create pyproject.toml with dependencies from requirements.txt and a dev optional group from requirements-dev.txt
2. Run uv sync to generate uv.lock
3. Remove requirements.txt and requirements-dev.txt
4. Update scripts/deploy.sh: pip install -r requirements.txt → uv sync --no-dev
5. Update .github/workflows/django-check.yaml and .github/workflows/check-gunicorn-config.yaml to install uv and use uv sync