Add Codespaces setup guide for new contributors

[Wendyshiro]

Summary

Adds instructions to run CARE in GitHub Codespaces without Docker or Python 3.13,
helping new contributors explore and test the app immediately.

Changes:

• Adds manual venv + pip setup guide
• Adds SQLite fallback for database configuration
• Documents Python 3.12 limitations in Codespaces

Future work: Add devcontainer for full Docker + Python 3.13 support.

Architecture Changes

No architectural changes. Documentation only.

Merge Checklist

• Documentation updated
• No tests required (docs-only change)
• Linting: N/A (Markdown only)