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.

documentation/backend/database.md

@@ -37,7 +37,7 @@ PostgreSQL database storing users, audiobooks, requests, downloads, configuratio
 - `id` (UUID PK), `plex_guid` (unique, external ID from Plex or Audiobookshelf), `plex_rating_key`
 - `title`, `author`, `narrator`, `summary`, `duration` (milliseconds), `year`, `user_rating` (0-10 scale)
 - **Universal identifiers:** `asin` (Audible ASIN), `isbn` (ISBN-10 or ISBN-13)
-- `file_path`, `thumb_url`, `plex_library_id`, `added_at`
+- `file_path`, `thumb_url`, `cached_library_cover_path` (local cached cover path), `plex_library_id`, `added_at`
 - `last_scanned_at`, `created_at`, `updated_at`
 - Indexes: `plex_guid`, `title`, `author`, `plex_library_id`, `asin`, `isbn`
 - **Purpose:** Universal library cache for both Plex and Audiobookshelf backends
@@ -45,6 +45,7 @@ PostgreSQL database storing users, audiobooks, requests, downloads, configuratio
   - **Plex:** ASIN extracted from Plex GUID (e.g., `com.plexapp.agents.audible://B00ABC123`) + stored in dedicated field
   - **Audiobookshelf:** ASIN/ISBN retrieved directly from ABS metadata + stored in dedicated fields
   - **Matching:** Prioritizes exact ASIN/ISBN matches (100% confidence) before fuzzy title/author matching
+- **Cached cover path:** Local path to cached library cover (e.g., `/app/cache/library/{hash}.jpg`), populated during scans
 
 ### Audiobooks
 - `id` (UUID PK), `audible_asin` (nullable), `title`, `author`, `narrator`, `description`