Quiz delete: detach questions to bank instead of deleting them

When deleting a quiz:
- Questions shared with other quizzes stay linked to those quizzes
- Questions exclusive to this quiz are detached (source_quiz_id cleared)
  and remain in the Question Bank as orphaned questions, not deleted
- This preserves the question bank integrity

Also improves manage.py CLI with extract, list-sections, and jobs commands

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/quizzes.py

@@ -441,8 +441,28 @@ def delete_quiz(
     db: Session = Depends(get_db),
     current_user: User = Depends(require_moderator),
 ):
+    """Delete quiz. Questions shared with other quizzes stay in the bank.
+    Questions only in this quiz are detached (source_quiz_id cleared) so they
+    remain in the bank instead of being deleted."""
+    from app.models.quiz_question_link import QuizQuestionLink
+
     quiz = db.query(Quiz).filter(Quiz.id == quiz_id).first()
     if not quiz:
         raise HTTPException(status_code=404, detail="Quiz not found")
-    db.delete(quiz)
+
+    # For each question linked to this quiz: if no other quiz uses it,
+    # detach it (clear source_quiz_id) so it stays as a bank-only question
+    links = db.query(QuizQuestionLink).filter(QuizQuestionLink.quiz_id == quiz_id).all()
+    for link in links:
+        other = db.query(QuizQuestionLink).filter(
+            QuizQuestionLink.question_id == link.question_id,
+            QuizQuestionLink.quiz_id != quiz_id,
+        ).count()
+        if other == 0:
+            # Detach from this quiz but keep in bank
+            db.query(QuestionModel).filter(QuestionModel.id == link.question_id).update(
+                {"quiz_id": None}  # source_quiz_id → null
+            )
+
+    db.delete(quiz)  # cascades quiz_question_links
     db.commit()