Introduce a comprehensive architecture summary document
detailing the modular, scalable, and minimal structure of
the Tux project. This document outlines the finalized
directory structure, rationale, extensibility model, and
how modules and custom modules work. It aims to provide
clarity and guidance for contributors and self-hosters,
ensuring a clean, scalable, and contributor-friendly
foundation. The document also addresses database
considerations and testing structure, supporting both
official features and self-hosted extensions. This
enhancement aligns with the architectural refactor
initiative to improve project structure and maintainability.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>