refactor: audit, split server.go, add unit tests, and fix latent bugs

- Remove dead code: browser cdp/content_scrape strategies, writer package,
  printUsage, downloadAndStoreCoverCLI in main.go
- Fix bugs: defer-in-loop in pocketbase deleteWhere, listAll() pagination
  hard cap removed, splitChapterTitle off-by-one in date extraction
- Split server.go (~1700 lines) into focused handler files:
  handlers_audio, handlers_browse, handlers_progress, handlers_ranking,
  handlers_scrape
- Export htmlutil.AttrVal/TextContent/ResolveURL; add storage/coverutil.go
  to consolidate duplicate helpers
- Flatten deeply nested conditionals: voices() early-return guards,
  ScrapeCatalogue next-link double attr scan, chapterNumberFromKey dead
  strings.Cut line, splitChapterTitle double-nested unit/suffix loop
- Add unit tests: htmlutil (9 funcs), novelfire ScrapeMetadata (3 cases),
  orchestrator Run (5 cases), storage chapterNumberFromKey/splitChapterTitle
  (22 cases); all pass with go build/vet/test clean

scraper/internal/storage/coverutil.go

@@ -0,0 +1,59 @@
+package storage
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+// DownloadAndStoreCover fetches the image at imageURL and stores it in the
+// store under key. Errors are logged but not returned — this is best-effort.
+// If the asset is already present the download is skipped.
+func DownloadAndStoreCover(store Store, log *slog.Logger, key, imageURL string) {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	// Skip if already stored.
+	if _, _, ok, _ := store.GetBrowseAsset(ctx, key); ok {
+		return
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, imageURL, nil)
+	if err != nil {
+		log.Warn("cover: build request failed", "key", key, "url", imageURL, "err", err)
+		return
+	}
+	req.Header.Set("User-Agent", "Mozilla/5.0 (compatible; libnovel-scraper/1.0)")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		log.Warn("cover: fetch failed", "key", key, "url", imageURL, "err", fmt.Errorf("%w", err))
+		return
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		log.Warn("cover: non-200 response", "key", key, "url", imageURL, "status", resp.StatusCode)
+		return
+	}
+
+	data, err := io.ReadAll(resp.Body)
+	if err != nil {
+		log.Warn("cover: read body failed", "key", key, "url", imageURL, "err", err)
+		return
+	}
+
+	contentType := resp.Header.Get("Content-Type")
+	if contentType == "" {
+		contentType = "image/jpeg"
+	}
+
+	if err := store.SaveBrowseAsset(ctx, key, data, contentType); err != nil {
+		log.Warn("cover: SaveBrowseAsset failed", "key", key, "err", err)
+		return
+	}
+	log.Debug("cover: stored", "key", key, "bytes", len(data))
+}