Three compounding issues caused the 4+ second load:
1. getAllRatings() ran sequentially after the first Promise.all group,
adding it unnecessarily to the critical path. Now runs in parallel
with listBooks/getVotedSlugs/getSavedSlugs (all 4 concurrent).
2. discovery_votes was fetched twice on every page load — once inside
getBooksForDiscovery (via getVotedSlugs) and again by getVotedBooks.
Fixed by caching getVotedSlugs results with a 30s TTL so the second
call hits cache instead of PocketBase.
3. getVotedSlugs and getSavedSlugs were always uncached, hitting
PocketBase on every navigation. Added short-TTL per-user Valkey
cache entries (voted: 30s, saved: 60s). Cache is invalidated
immediately after each write (upsertDiscoveryVote, clearDiscoveryVotes,
undoDiscoveryVote, saveBook) so stale data is never served.
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