Add CONTRIBUTING.md with setup guide

[l2dnjsrud]

Description

A CONTRIBUTING.md file would help new contributors get started quickly.

Should include

• Development setup (backend + frontend)
• How to run tests
• Code style guidelines
• PR process
• Architecture overview (which files do what)

Difficulty

Beginner-friendly. Documentation only.

[Kush-Saharan]

Hey! I would like to work on this issue if its open for contributions.

[l2dnjsrud]

Welcome! Yes, this issue is open — feel free to pick it up.

A few pointers to get started:

• The backend is in backend/ (Python/FastAPI), frontend in frontend/ (Vue 3/TypeScript)
• Run cd backend && pip install -r requirements.txt && uvicorn app.main:app --reload for the backend
• Run cd frontend && npm install && npm run dev for the frontend
• Tests: cd backend && pytest tests/ -v

For CONTRIBUTING.md, it would be helpful to cover:

1. Dev setup (backend + frontend)
2. How to run tests
3. PR process (fork → branch → PR)
4. Brief architecture overview (what each directory does)

Feel free to open a draft PR early if you want feedback along the way. Thanks for contributing!