fix: reduce log noise during catalogue/book scrapes

Demote per-book and per-chapter-list-page Info logs to Debug — these
fire hundreds of times per catalogue run and drown out meaningful signals:
- orchestrator: RunBook starting (per book)
- metadata saved (per book)
- chapter list fetched (per book)
- scraping chapter list page N (per pagination page per book)

The 'book scrape finished' summary log (with scraped/skipped/errors
counters) remains at Info — it is the useful signal per book.

backend/internal/novelfire/scraper.go

@@ -241,7 +241,7 @@ func (s *Scraper) ScrapeChapterList(ctx context.Context, bookURL string, upTo in
 		}
 
 		pageURL := fmt.Sprintf("%s?page=%d", baseChapterURL, page)
-		s.log.Info("scraping chapter list", "page", page, "url", pageURL)
+		s.log.Debug("scraping chapter list", "page", page, "url", pageURL)
 
 		raw, err := retryGet(ctx, s.log, s.client, pageURL, 9, 6*time.Second)
 		if err != nil {