Fix outdated backend README (T431214)

[priyankar1434]

What's this PR about?

The backend README was pretty outdated — it said the stack was Flask/Python/PostgreSQL, but the actual code runs on Node.js/Express/MongoDB. It also had a bunch of duplicate sections (Project Structure, API Endpoints, Testing, and Deployment were all repeated twice), which made it confusing to read.

What I changed

• Fixed the Tech Stack section to actually match what's in the code — Node.js, Express, MongoDB/Mongoose, JWT for auth
• Rewrote the Project Structure to match the real backend/src/ folder layout
• Replaced the API Endpoints list with the actual routes — went through each file in backend/src/routes/ to make sure every path and method is correct
• Removed the duplicate sections
• Updated the setup instructions (installation, env variables, testing) to match what's really in package.json

How I verified it

Went through backend/src/routes/*.js and backend/src/server.js directly and cross-checked every endpoint and route prefix instead of guessing.

Related task

Closes T431214: https://phabricator.wikimedia.org/T431214

Checklist

• README now reflects the actual stack, scripts, and endpoints
• Duplicate content removed

[vercel]

@priyankar1434 is attempting to deploy a commit to the Tiisu's projects Team on Vercel.

A member of the Team first needs to authorize it.