feat: add Copier template for Python projects

  - Python 3.13 with uv package manager and uv_build backend
  - Optional fullstack frontend (React 19 + Vite 7 + Tailwind v4)
  - Claude Code configuration with marketplace plugins
  - GitHub Actions CI, pre-commit hooks, gitleaks
  - Comprehensive .gitignore with secret protection

  Co-Authored-By: Claude Opus 4.5 <[email protected]>