Adds docs/architecture.d2 and docs/architecture.mermaid.md showing the
docker-compose-new.yml service topology — storage, application, init
containers, and external dependencies with annotated connections.
Also includes the rendered docs/architecture.svg (D2 output).
View live: d2 --watch docs/architecture.d2
View in Gitea: navigate to docs/architecture.mermaid.md in the web UI.
