Feat/setup vitest testing, update ESLint and synchronise with main

[Pinkish-Warrior]

This Pull Request introduces comprehensive unit testing for the backend, updates the ESLint configuration for typed linting, and synchronises the feature branch with the latest changes from main.
Key Changes:

• Vitest Integration:
  • Adds Vitest as the primary unit testing framework for the backend.
  • Includes initial test files for llm.service.ts and ai.controller.ts, along with basic setup and mocks.
  • Configures vitest.config.ts for proper test execution and coverage.
• ESLint Configuration Update:
  • Updates eslint.config.js and tsconfig.eslint.json to enable typed linting across the project, including test files.
  • Resolves ESLint parsing errors in test files.
  • Fixes a no-undef error for Express types in upload.ts.
• Documentation:
  • Updates README.md and adds docs/VITEST.md and docs/LINTS.md to reflect the new testing and linting setup and
    available scripts.
• Main Branch Synchronisation:
  • Successfully merged the latest main branch changes (including recent linting infrastructure updates) into this
    feature branch, resolving all conflicts locally.

Why these changes?
This work significantly enhances code quality and reliability by:

• Enabling robust unit testing of core backend services and controllers.
• Ensuring consistent code style and preventing common errors through updated ESLint rules.
• Keeping the codebase up-to-date with the latest main changes, minimising future integration issues.

Areas for Review:

• Please review the new test files in backend/src/tests for coverage and correctness.
• Review the updated ESLint configuration to ensure it aligns with project standards.
• Verify the README.md, docs/VITEST.md, and docs/LINTS.md updates.

[MariShopen]

all good for me