docs(app-backend): add backend system architecture overview

[LoopyB]

Summary

This PR adds backend system architecture documentation for the SecureShift app-backend.

The document provides a structured overview of:

• backend system context and major components
• request and data flow
• route, middleware, controller, service, model, adapter, and utility responsibilities
• current vs target backend layering
• RBAC and access-control considerations
• data ownership and tenant-scoping risks
• current API surface and mounted route groups
• recent backend improvements and architecture impact
• current gaps, architectural debt, and recommended roadmap

Purpose

The goal is to support backend onboarding, handover, maintainability, and future development planning.

This documentation is intended as a living technical reference for future SecureShift backend contributors. It captures current backend structure, known risks, and recommended next steps, including service-layer consolidation, RBAC cleanup, tenant-aware access control, regression testing, and future analytics readiness.

Notes

This PR is documentation-only and does not change runtime backend behaviour.