8. Add `POST /api/v1/admin/courses/:id/modules/:moduleId/quizzes/:quizId/questions` endpoint

[DeFiVC]

What: Add an admin endpoint for adding questions to an existing quiz.

Why: Admins need to manually add or modify quiz questions. Currently quizzes are generated as a whole by the AI service.

Scope: Create endpoint that adds a question to an existing quiz. Validate question structure (prompt, options, correct index). Update the quiz's questions JSONB array.

Acceptance criteria:

• Admin can add questions to existing quizzes
• Question structure is validated
• Quiz is updated atomically
• Changes are logged in audit logs

Technical context: src/modules/quizzes/quiz.service.ts — quiz operations. src/database/schema.ts:88-109 — quizzes table.

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