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/scraper/interfaces.go

@@ -120,6 +120,16 @@ type RankingProvider interface {
 	ScrapeRanking(ctx context.Context, maxPages int) (<-chan BookMeta, <-chan error)
 }
 
+// RankingPageCacher persists and retrieves raw HTML for individual ranking pages.
+// Implementations (e.g. writer.Writer) store files on disk so that a
+// subsequent ScrapeRanking call can serve cached HTML without a network round-trip.
+type RankingPageCacher interface {
+	// WriteRankingPageCache stores the raw HTML string for the given page number.
+	WriteRankingPageCache(page int, html string) error
+	// ReadRankingPageCache returns the cached HTML for page, or ("", nil) on a miss.
+	ReadRankingPageCache(page int) (string, error)
+}
+
 // NovelScraper is the full interface that a concrete novel source must implement.
 // It composes all four provider interfaces.
 type NovelScraper interface {