Fix Mermaid rendering compatibility in README

[lraigosov]

Summary

• fix Mermaid diagram compatibility in README architecture section
• keep architecture relationships unchanged while simplifying node definitions

Validation performed

• reviewed commit scope (README.md only)
• validated compose rendering on Ubuntu-24.04 for PostgreSQL, Portainer, and LibreChat stacks
• confirmed local branch is clean and aligned with origin/dev before opening PR

Review notes

• verify Mermaid block renders correctly on GitHub rich markdown viewer
• confirm no service behavior changes were introduced beyond documentation
• merge requested without deleting branches