Add backend integration test setup with vitest

[DevalGarg129]

Summary

Added backend integration testing setup for the Express + Mongoose backend using Vitest, Supertest, and mongodb-memory-server.

What Changed

Testing Infrastructure

• Added Vitest configuration for backend integration testing
• Added isolated in-memory MongoDB test setup
• Added reusable backend test environment
• Added npm test command support

Backend Integration Tests

Implemented integration tests for:

• Auth route protection flow
• Friend request flow
• Friend request processing flow
• Direct message unauthorized send flow
• Direct message unauthorized fetch flow

Developer Experience

• Tests now run without requiring production MongoDB Atlas credentials
• Added isolated database cleanup between test runs
• Added contributor-friendly backend testing workflow

Why

This improves backend reliability and makes future contributions safer by providing a consistent integration testing setup for maintainers and contributors.

Testing

Run backend tests with:

cd server
npm test

All tests pass successfully using an isolated in-memory MongoDB instance.

<issue: 23>

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