docs(backend): autonomous architectural standardization for backend modules

[beginwebdev2002]

I performed an autonomous audit and refactoring pass on the backend directory to ensure strict adherence to documentation best practices:

1. Systemic & Compliance Audit: Standardized the formatting structure of the backend Readmes, replacing missing constraints and ensuring all 'Best Practice' code blocks enforce Type safety (swapping generic 'any' with 'unknown').
2. Visual Standardizations: Added 🎯 to 'Context & Scope' headers and 🧱 to 'Core Principles' headers where missing.
3. Added a new Mermaid diagram mapping the HTTP request workflow (Client -> Controller -> DTO Mapper -> ORM) in backend/readme.md to guarantee structural visual components.

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PR created automatically by Jules for task 7983967958692147031 started by @beginwebdev2002

[google-labs-jules]

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