Adds atomic counters for books_found, chapters_scraped, chapters_skipped, and errors. The new OnProgress callback fires after each counter update and once more at the end of Run, giving callers a live progress feed.
- Add storage/hybrid.go: HybridStore composing PocketBase + MinIO backends - Rewrite orchestrator to accept storage.Store instead of *writer.Writer - Replace *writer.Writer with storage.Store in server.go and ui.go - Wire audio cache, reading progress, chapter reads/writes through store - Add rankingCacheAdapter in main.go to bridge context-free RankingPageCacher interface to HybridStore's context-aware methods
