The home, library, and /books routes were fetching all 15k books from
PocketBase on every SSR request (31 sequential HTTP calls per request).
Changes:
- Add src/lib/server/cache.ts: generic Valkey JSON cache
- Add getBooksBySlugs(): single PB query fetching only requested slugs,
with fallback to the 5-min Valkey cache populated by listBooks()
- listBooks(): now caches results in Valkey for 5 min (safety net for
admin routes that still need the full list)
- Home + /api/home: replaced listBooks()+filter with getBooksBySlugs()
on progress slugs only — typically 1 PB request instead of 31
- /books + /api/library: same pattern using progress+saved slug union
