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/server/handlers_scrape.go

@@ -33,6 +33,29 @@ func (s *Server) handleScrapeBook(w http.ResponseWriter, r *http.Request) {
 	s.runAsync(w, cfg)
 }
 
+// handleScrapeBookRange handles POST /api/scrape/book/range.
+// Body: {"url": "...", "from": N, "to": M}
+// Scrapes only chapters in the range [from, to] (inclusive).
+// from=0 means "start from chapter 1"; to=0 means "no upper limit".
+func (s *Server) handleScrapeBookRange(w http.ResponseWriter, r *http.Request) {
+	var body struct {
+		URL  string `json:"url"`
+		From int    `json:"from"`
+		To   int    `json:"to"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil || body.URL == "" {
+		http.Error(w, `{"error":"request body must be JSON with \"url\" field"}`, http.StatusBadRequest)
+		return
+	}
+
+	cfg := s.oCfg
+	cfg.SingleBookURL = body.URL
+	cfg.FromChapter = body.From
+	cfg.ToChapter = body.To
+
+	s.runAsync(w, cfg)
+}
+
 // runAsync launches an orchestrator in the background and returns 202 Accepted.
 // Only one scrape job runs at a time; concurrent requests receive 409 Conflict.
 func (s *Server) runAsync(w http.ResponseWriter, cfg orchestrator.Config) {