feat(domain): add library types, LibrarySyncAdapter, ILibraryRepository, IAppSettingsRepository; extend MediaItem with thumbnail_url and collection_id

k-tv-backend/domain/src/entities.rs

@@ -346,6 +346,10 @@ pub struct MediaItem {
     pub season_number: Option<u32>,
     /// For episodes: episode number within the season (1-based).
     pub episode_number: Option<u32>,
+    /// Provider-served thumbnail image URL, populated if available.
+    pub thumbnail_url: Option<String>,
+    /// Provider-specific collection this item belongs to.
+    pub collection_id: Option<String>,
 }
 
 /// A fully resolved 7-day broadcast program for one channel.