Fix display: update n_chunks after chunk boundary filtering

The 'Chunk X/5' message was showing 5 even when only 2 chunks were
being processed (pages 1-50 and 51-55). n_chunks was not updated
after filtering chunks to stay before answer_section_start.
Cosmetic fix — the actual extraction was already correct.

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

backend/app/tasks/quiz_tasks.py

@@ -130,6 +130,7 @@ def extract_quiz(
             chunks = [(s, min(e, q_end)) for s, e in chunks if s <= q_end]
             if not chunks:
                 chunks = [(section.start_page, q_end)]
+            n_chunks = len(chunks)  # update display count after filtering
 
         all_valid_questions = []
         all_skipped = []