Adds FromChapter/ToChapter fields to orchestrator.Config and skips out-of-range
chapters in processBook. Exposes POST /scrape/book/range Go endpoint and a
matching UI proxy at /api/scrape/range. The book detail page now shows admins
a range input (from/to chapter) and a per-chapter 'scrape from here up' button.
Adds atomic counters for books_found, chapters_scraped, chapters_skipped,
and errors. The new OnProgress callback fires after each counter update
and once more at the end of Run, giving callers a live progress feed.
