feat: switch ranking storage from markdown to JSON, fix round-trip test

- WriteRanking now writes ranking.json via json.MarshalIndent (replaces markdown table)
- ReadRankingItems uses json.Unmarshal (no fragile pipe-split parsing)
- ReadRanking() (raw string reader) removed; handleRankingView uses ReadRankingItems
- handleRankingView renders a <pre> JSON block instead of goldmark markdown
- RankingItem gains json: struct tags; slug and genres now survive the round-trip exactly
- TestWriteRanking_RoundTrip updated: checks ranking.json, verifies Slug/Genres/SourceURL
- Add RankingPageCacher interface and per-page HTML disk cache support in scraper

scraper/internal/server/ui.go

@@ -933,31 +933,29 @@ const rankingViewTmpl = `
 
   <h1 class="text-3xl font-bold text-zinc-100 mb-6">Ranking Data</h1>
 
-  <div class="prose prose-invert max-w-none">
-    {{.HTML}}
-  </div>
+  <pre class="text-xs text-zinc-300 bg-zinc-900 border border-zinc-700 rounded-xl p-4 overflow-x-auto whitespace-pre-wrap break-words">{{.JSON}}</pre>
 </div>`
 
 func (s *Server) handleRankingView(w http.ResponseWriter, r *http.Request) {
-	markdown, err := s.writer.ReadRanking()
+	items, err := s.writer.ReadRankingItems()
 	if err != nil {
 		http.Error(w, "failed to read ranking: "+err.Error(), http.StatusInternalServerError)
 		return
 	}
-	if markdown == "" {
+	if len(items) == 0 {
 		http.NotFound(w, r)
 		return
 	}
 
-	var htmlBuf bytes.Buffer
-	if err := md.Convert([]byte(markdown), &htmlBuf); err != nil {
-		http.Error(w, "markdown render error: "+err.Error(), http.StatusInternalServerError)
+	pretty, err := json.MarshalIndent(items, "", "  ")
+	if err != nil {
+		http.Error(w, "json marshal error: "+err.Error(), http.StatusInternalServerError)
 		return
 	}
 
 	t := template.Must(template.New("rankingView").Parse(rankingViewTmpl))
 	var buf bytes.Buffer
-	_ = t.Execute(&buf, struct{ HTML template.HTML }{HTML: template.HTML(htmlBuf.String())})
+	_ = t.Execute(&buf, struct{ JSON string }{JSON: string(pretty)})
 
 	s.respond(w, r, "Ranking Data", buf.String())
 }