feat(cli): wire catalog URL to Create-Python-App/cpa-templates

[ulises-jeremias]

Summary

Point create-awesome-python-app catalog fetch to cpa-templates templates.json (raw GitHub URL), with CPA_CATALOG_URL override.

Acceptance criteria

• Default URL: raw.githubusercontent.com/Create-Python-App/cpa-templates/main/templates.json
• CPA_CATALOG_URL env override
• Offline/fixture fallback preserved
• Tests updated

Closes tracking for #44 when cpa-templates is live.