[Anshuu2004]

What does this PR do?

Fixes the co-organizer invite flow so bad input gets a proper error instead of a 500, and the sidebar actually lists people.

Backend (inviteController.ts):

• no email, a blank one, or an invalid one now gives 400
• inviting yourself, someone already invited, or someone already an organizer gives 409 with its own message. I check inviteEmail as well as the user, because the @@unique([userId, quizId]) constraint doesn't catch repeat invites to an email that has no account yet (userId is null there)
• emails are trimmed and lower cased when stored, and the match on accept ignores case, so old rows saved in mixed case still work
• an organizer who opens a second invite link gets a 409 instead of a 500, and I delete the leftover pending row so it doesn't sit in the list
• Prisma P2002 is caught as a fallback on both endpoints

Frontend (QuizBuilder.tsx):

• the invite form shows the error text from the API instead of always "Failed to invite."
• the sidebar was reading quiz.organizers but the API sends quizOrganizers, so it was always empty. Fixed, and it shows OWNER / ACCEPTED / PENDING
• the quiz reloads after a successful invite so the pending one shows up

http-server had no tests so I added vitest the way ws-server has it, one small test file for the email check, and a test step in ci.yml. Same setup lines as my other PR for #211, so whichever merges first the other should apply cleanly.

Related issue

Closes #214

Type of change

• Bug fix
• New feature
• Documentation
• Refactoring
• Performance
• Tests
• Build / CI / deployment

Which services does this touch?

• apps/frontend
• apps/http-server
• apps/ws-server
• apps/genAI
• packages/db (schema or Prisma changes)
• Docker / CI

How did you test this?

• pnpm --filter http-server run test: 14 pass
• pnpm --filter frontend run test: 75 pass, 3 of them new (sidebar and the error message)
• ran http-server on a throwaway Postgres with all migrations and hit the endpoints before and after. Before: no email 500, blank or invalid email 200, inviting yourself 200, inviting the same user twice 500, second invite link for an existing organizer 500, a Bob@Example.com account could not accept an invite sent to bob@example.com. After: 400 for missing, blank and invalid email, 409 for self invite, 409 "Already invited" (also for a different letter case and for an email with no account), 409 "Already an organizer", 409 on the second invite link, and the mixed case account accepts fine
• ran the frontend with vite against that server and looked at the sidebar in a browser (screenshot above), one session only
• I did not try docker compose up

Checklist

• I ran pnpm run lint
• I ran pnpm run check-types
• I ran pnpm run build
• The app still starts with docker-compose up
• I updated documentation if this changes behaviour or setup
• I did not commit any .env file, API key or other secret

pnpm run lint still fails in the frontend because of errors that were already on main (18, mostly no-explicit-any), my change adds none. check-types has no script for http-server or the frontend, so I also ran tsc on both and it passes. No schema change, no migration.