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/novelfire/ranking_test.go

@@ -164,9 +164,9 @@ func TestWriteRanking_RoundTrip(t *testing.T) {
 		t.Fatalf("WriteRanking failed: %v", err)
 	}
 
-	rankingFile := filepath.Join(dir, "ranking.md")
+	rankingFile := filepath.Join(dir, "ranking.json")
 	if _, err := os.Stat(rankingFile); err != nil {
-		t.Fatalf("ranking.md not created: %v", err)
+		t.Fatalf("ranking.json not created: %v", err)
 	}
 
 	got, err := w.ReadRankingItems()
@@ -180,12 +180,27 @@ func TestWriteRanking_RoundTrip(t *testing.T) {
 		if got[i].Rank != want.Rank {
 			t.Errorf("item[%d].Rank = %d, want %d", i, got[i].Rank, want.Rank)
 		}
+		if got[i].Slug != want.Slug {
+			t.Errorf("item[%d].Slug = %q, want %q", i, got[i].Slug, want.Slug)
+		}
 		if got[i].Title != want.Title {
 			t.Errorf("item[%d].Title = %q, want %q", i, got[i].Title, want.Title)
 		}
 		if got[i].Status != want.Status {
 			t.Errorf("item[%d].Status = %q, want %q", i, got[i].Status, want.Status)
 		}
+		if len(got[i].Genres) != len(want.Genres) {
+			t.Errorf("item[%d].Genres len = %d, want %d", i, len(got[i].Genres), len(want.Genres))
+		} else {
+			for j, g := range want.Genres {
+				if got[i].Genres[j] != g {
+					t.Errorf("item[%d].Genres[%d] = %q, want %q", i, j, got[i].Genres[j], g)
+				}
+			}
+		}
+		if got[i].SourceURL != want.SourceURL {
+			t.Errorf("item[%d].SourceURL = %q, want %q", i, got[i].SourceURL, want.SourceURL)
+		}
 	}
 }