Use uv for package management

Use `uv` as the default Python package manager - it's the preferred choice:
- 10-100x faster than pip/poetry
- Built-in virtual environment management
- Lockfile support (uv.lock) for reproducible builds
- Compatible with pyproject.toml and requirements.txt
- Rust-based, single binary, no Python dependency

```bash
# Install uv
curl -LsSf https://astral.sh/uv/install.sh | sh

# Create new project
uv init myproject
cd myproject

# Add dependencies
uv add fastapi sqlalchemy pydantic

# Add dev dependencies
uv add --dev pytest ruff mypy

# Run commands
uv run python main.py
uv run pytest
```

NEVER use pip, poetry, or pipenv for new projects - always use uv.