Add relevance ranking with Bayesian scoring and sort toggle

Backend:
- Compute relevance score per book at export time using Bayesian
  weighted rating * log2(ratingCount) — balances quality and popularity
- Sort exported JSON by relevance score descending (default)
- Include relevanceScore in exported data for frontend sort toggle
- Extend AI narration filter to catch voice replicas/clones

Frontend:
- Add Relevance/Release Date toggle in toolbar
- Default to relevance ordering (flat ranked grid)
- Release Date mode preserves existing month-grouped layout
- Add relevanceScore, rating, ratingCount to Book type

Closes #38