Docs: add backend architecture reference and update contributor links #578

[Apoorv-Raj777]

Closes #578

Description

This PR creates a dedicated, contributor-oriented backend architecture guide under docs/backend-architecture.md and updates related reference links in existing documentation files.

Key updates include:

• Created docs/backend-architecture.md: A comprehensive architectural map containing:
  • A Module Map details the responsibility of all 32+ files inside backend/secuscan/.
  • An End-to-End Scan Execution Flow visualised with a Mermaid sequence diagram.
  • A Layer-by-Layer Reference highlighting configuration, bootstrap, execution engine sandbox boundaries, persistence, native scanners, validation rules, risk scoring, reporting, and workflows behavior.
  • Key Data Models defining execution lifecycles (TaskStatus, ScanPhase, Finding, and ExecutionContext).
  • Extension Points explaining how to contribute new API routes, native scanners, or plugins.
  • Related Docs linking relevant developer instructions.
• Updated CONTRIBUTING.md: Linked the backend architecture reference under "Good First Contribution Areas".
• Updated docs/API.md: Appended a "See Also" section referencing the new architecture document.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Verified that all relative paths and markdown links resolve correctly.
• Checked git diff and git status to ensure only .md files were modified, leaving functional code unaltered.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

Reviewed. Docs-only backend architecture/contributor link updates are scoped and consistent enough to merge.