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/server/integration_test.go

@@ -335,6 +335,9 @@ func TestServer_PresignChapter_NotFound(t *testing.T) {
 	// MinIO presign on a non-existent key returns an error; server returns 500.
 	// (Some MinIO versions return a valid presigned URL anyway, which is also acceptable.)
 	t.Logf("presign non-existent chapter status: %d", resp.StatusCode)
+	if resp.StatusCode != http.StatusInternalServerError && resp.StatusCode != http.StatusOK {
+		t.Errorf("status = %d, want 500 or 200", resp.StatusCode)
+	}
 }
 
 // TestServer_ChapterText writes a chapter and verifies
@@ -395,13 +398,6 @@ func mapKeys(m map[string]interface{}) []string {
 	return keys
 }
 
-func min(a, b int) int {
-	if a < b {
-		return a
-	}
-	return b
-}
-
 // cookieJar is a minimal http.CookieJar that stores cookies by host.
 type cookieJar struct {
 	cookies map[string][]*http.Cookie