Scrum 33 backend quizzes

[Ligo-code]

Quiz Generation Feature

Add AI-powered quiz generation from study resources with complete CRUD operations.

Added:

• 3 models: Quiz, QuizQuestion, QuizAttempt
• AI service with Gemini API integration (3-50 questions)
• 7 protected REST endpoints with rate limiting (20/15min)
• MongoDB transactions with rollback support

Endpoints:
Generate, list, take, submit, get results, delete quizzes

I hadn't studied at all(

After studing, I got 100% Yey!!!:

Now, I'm really proud of this result!

And I don't need this quiz anymore.

[AidaBur]

I tested quiz generation with different question counts.
Generation works reliably for 5–10 questions, but starts failing around 15+, even though backend validation allows up to 50.
This seems to be an LLM limitation rather than a backend issue. Given that we agreed 10 questions is the comfortable default, it probably makes sense to enforce this limit on the frontend to avoid unnecessary generation failures.

Overall, the quiz flow looks solid.
As a minor improvement, error response formats could be unified for easier frontend handling (some return { success: false, error: "..." } while others return { error: "..." }). This is not blocking.

Great work overall! LGTM and good to go 👍