fix: make ranking refresh async and use JS-rendered client for ScrapeRanking

Two bugs caused the 'Refresh Rankings' button to silently fail in production:

1. ScrapeRanking was using the plain HTTP client (s.client) instead of the
   browserless content client (s.urlClient). The /ranking page requires
   JavaScript rendering, so a plain fetch returned HTML without any novel
   entries. Now uses s.urlClient so the page is fully rendered before scraping.

2. handleRankingRefresh was synchronous, holding the HTTP connection open for
   up to 60 s while scraping. Reverse proxies and HTMX timeouts closed the
   connection before the scrape finished. Rewritten to the same async pattern
   used for book scraping: POST /ranking/refresh returns immediately with a
   polling badge; the browser polls GET /ui/ranking/status every 3 s; when
   the goroutine finishes the status endpoint sends HX-Redirect to /ranking.

scraper/internal/server/server.go

@@ -24,15 +24,16 @@ import (
 
 // Server wraps an HTTP mux with the scraping endpoints.
 type Server struct {
-	addr        string
-	oCfg        orchestrator.Config
-	novel       scraper.NovelScraper
-	log         *slog.Logger
-	writer      *writer.Writer
-	mu          sync.Mutex
-	running     bool
-	kokoroURL   string // Kokoro-FastAPI base URL, e.g. http://kokoro:8880
-	kokoroVoice string // default voice, e.g. af_bella
+	addr           string
+	oCfg           orchestrator.Config
+	novel          scraper.NovelScraper
+	log            *slog.Logger
+	writer         *writer.Writer
+	mu             sync.Mutex
+	running        bool
+	rankingRunning bool
+	kokoroURL      string // Kokoro-FastAPI base URL, e.g. http://kokoro:8880
+	kokoroVoice    string // default voice, e.g. af_bella
 }
 
 // New creates a new Server.
@@ -65,6 +66,7 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("GET /books/{slug}/chapters-page", s.handleBookChaptersPage)
 	mux.HandleFunc("POST /ui/scrape/book", s.handleUIScrapeBook)
 	mux.HandleFunc("GET /ui/scrape/status", s.handleUIScrapeStatus)
+	mux.HandleFunc("GET /ui/ranking/status", s.handleRankingStatus)
 	// Plain-text chapter content for browser-side TTS
 	mux.HandleFunc("GET /ui/chapter-text/{slug}/{n}", s.handleChapterText)