Rewrite the README to better reflect the current state and scope of the project.

- Add CI and stack badges for better visibility
- Clarify academic context and pedagogical objectives
- Document implemented, ongoing, and planned features in detail
- Describe application architecture and data model
- Document testing strategy, security, accessibility, and RGPD compliance
- Improve installation, setup, and onboarding instructions
- Add CI/CD, deployment context, and contribution conventions

This update turns the README into a comprehensive technical and pedagogical reference,
suitable for evaluation, onboarding, and long-term maintenance.