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/go.sum

@@ -6,8 +6,6 @@ github.com/go-ini/ini v1.67.0 h1:z6ZrTEZqSWOTyH2FlglNbNgARyHG8oLW9gMELqKr06A=
 github.com/go-ini/ini v1.67.0/go.mod h1:ByCAeIL28uOIIG0E3PJtZPDL8WnHpFKFOtgjp+3Ies8=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
-github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
-github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
 github.com/klauspost/compress v1.18.2 h1:iiPHWW0YrcFgpBYhsA6D1+fqHssJscY/Tm/y2Uqnapk=
 github.com/klauspost/compress v1.18.2/go.mod h1:R0h/fSBs8DE4ENlcrlib3PsXS61voFxhIs2DeRhCvJ4=
 github.com/klauspost/cpuid/v2 v2.0.1/go.mod h1:FInQzS24/EEf25PyTYn52gqo7WaD8xa0213Md/qVLRg=