feat: ranking page pagination, popular URL, and per-page HTML disk cache

- Switch ScrapeRanking to novelfire.net/genre-all/sort-popular URL and updated DOM selectors (div.novel-item, h3.novel-title, div.genres)
- Replace 5 hardcoded refresh buttons with dynamic 100-page paginator (smart ellipsis via rankingPageNums)
- Add RankingPageCacher interface and writer methods to cache raw HTML per page under static/books/_ranking_cache/page-N.html
- ScrapeRanking serves from disk cache on hit and writes to cache on miss, skipping Browserless round-trip
- Thread writer as PageCacher through novelfire.New and main.go
- Add TestScrapeRanking_CacheHit and TestScrapeRanking_CacheMiss tests

scraper/internal/writer/writer.go

@@ -403,6 +403,57 @@ func (w *Writer) rankingPath() string {
 	return filepath.Join(w.root, "ranking.md")
 }
 
+// ─── Ranking page HTML cache ──────────────────────────────────────────────────
+
+// rankingCacheDir returns the directory that stores per-page HTML caches.
+func (w *Writer) rankingCacheDir() string {
+	return filepath.Join(w.root, "_ranking_cache")
+}
+
+// rankingPageCachePath returns the path for a cached ranking page HTML file.
+func (w *Writer) rankingPageCachePath(page int) string {
+	return filepath.Join(w.rankingCacheDir(), fmt.Sprintf("page-%d.html", page))
+}
+
+// WriteRankingPageCache persists raw HTML for the given ranking page number.
+func (w *Writer) WriteRankingPageCache(page int, html string) error {
+	dir := w.rankingCacheDir()
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("writer: mkdir ranking cache %s: %w", dir, err)
+	}
+	path := w.rankingPageCachePath(page)
+	if err := os.WriteFile(path, []byte(html), 0o644); err != nil {
+		return fmt.Errorf("writer: write ranking page cache %s: %w", path, err)
+	}
+	return nil
+}
+
+// ReadRankingPageCache reads the cached HTML for the given ranking page.
+// Returns ("", nil) when no cache file exists yet.
+func (w *Writer) ReadRankingPageCache(page int) (string, error) {
+	data, err := os.ReadFile(w.rankingPageCachePath(page))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", nil
+		}
+		return "", fmt.Errorf("writer: read ranking page cache page %d: %w", page, err)
+	}
+	return string(data), nil
+}
+
+// RankingPageCacheInfo returns os.FileInfo for a cached ranking page file.
+// Returns (nil, nil) when the file does not exist.
+func (w *Writer) RankingPageCacheInfo(page int) (os.FileInfo, error) {
+	info, err := os.Stat(w.rankingPageCachePath(page))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	return info, nil
+}
+
 // bookDir returns the root directory for a book slug.
 func (w *Writer) bookDir(slug string) string {
 	return filepath.Join(w.root, slug)