feat(scrape-range): add chapter range scraping for admin users

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.

scraper/internal/orchestrator/orchestrator.go

@@ -45,6 +45,14 @@ type Config struct {
 	// that one book instead of walking the full catalogue.
 	SingleBookURL string
 
+	// FromChapter, when > 0, skips chapters with number < FromChapter.
+	// Only effective in single-book mode.
+	FromChapter int
+
+	// ToChapter, when > 0, skips chapters with number > ToChapter.
+	// Only effective in single-book mode.  0 means "no upper limit".
+	ToChapter int
+
 	// OnProgress is called periodically with the current progress counters.
 	// It is always called on completion (success or failure). May be nil.
 	OnProgress func(p Progress)
@@ -204,6 +212,15 @@ func (o *Orchestrator) Run(ctx context.Context) error {
 
 		// Enqueue chapter jobs.
 		for _, ref := range refs {
+			// Apply chapter range filter (only in single-book mode when set).
+			if o.cfg.FromChapter > 0 && ref.Number < o.cfg.FromChapter {
+				chaptersSkipped.Add(1)
+				continue
+			}
+			if o.cfg.ToChapter > 0 && ref.Number > o.cfg.ToChapter {
+				chaptersSkipped.Add(1)
+				continue
+			}
 			select {
 			case <-ctx.Done():
 				return