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;

prisma/schema.prisma

@@ -45,7 +45,8 @@ model User {
   registrationStatus String? @default("approved") @map("registration_status") // 'pending_approval' | 'approved' | 'rejected'
 
   // BookDate per-user preferences
-  bookDateLibraryScope       String? @default("full") @map("bookdate_library_scope") // 'full' | 'rated'
+  bookDateLibraryScope       String? @default("full") @map("bookdate_library_scope") // 'full' | 'rated' | 'favorites'
+  bookDateFavoriteBookIds    String? @map("bookdate_favorite_book_ids") @db.Text // JSON array of PlexLibrary IDs (max 25)
   bookDateCustomPrompt       String? @map("bookdate_custom_prompt") @db.Text
   bookDateOnboardingComplete Boolean @default(false) @map("bookdate_onboarding_complete")
 
@@ -130,8 +131,9 @@ model PlexLibrary {
   isbn String? // ISBN (10 or 13) - for additional matching capability
 
   // File information
-  filePath String? @map("file_path") @db.Text
-  thumbUrl String? @map("thumb_url") @db.Text // Plex thumbnail URL
+  filePath            String? @map("file_path") @db.Text
+  thumbUrl            String? @map("thumb_url") @db.Text // Plex thumbnail URL
+  cachedLibraryCoverPath String? @map("cached_library_cover_path") @db.Text // Local path to cached library cover image
 
   // Plex metadata
   plexLibraryId String    @map("plex_library_id") // Which Plex library contains this