Add BookDate card stack animations and thumbnail caching

Implements pure CSS card stack animations for BookDate recommendations, including smooth exit and advance transitions. Adds local caching of library cover thumbnails during scans, updates database schema and API to serve cached covers, and enhances BookDate to support 'favorites' scope with a book picker modal. Updates admin settings validation logic for Prowlarr, improves indexer state management, and documents new features and backend changes.

prisma/migrations/20260120000000_add_cached_library_cover_path/migration.sql

@@ -0,0 +1,2 @@
+-- AddCachedLibraryCoverPath
+ALTER TABLE "plex_library" ADD COLUMN "cached_library_cover_path" TEXT;