Detect and filter AI-narrated (Virtual Voice) books

- Add `isAiNarrated()` detection in Audible fetcher: flags books with
  "Virtual Voice" narrator or no narrator at all
- Add `is_ai_narrated` to BookRow and persist it in upsert queries
- Set export filter qualityScoreOverride to 99 to effectively hard-filter
  all AI-narrated books (these are ~30-50% of results and nearly all
  low-quality false positives)

Closes #10