kill LibraryItem, unify to MediaItem; decouple schedule engine from providers

ADR-0001: MediaItem absorbs LibraryItem fields (provider_id, external_id,
collection_name, collection_type, synced_at, role/MediaRole).
LibraryItem + LibraryItemRow deleted. All ports/adapters/tests updated.

ADR-0002: schedule engine takes LibraryQuery instead of IProviderRegistry.
Algorithmic blocks query library via search(), manual blocks via get_by_id().
get_stream_url removed from engine; provider_registry moved to ScheduleDeps
for playback-time stream URL resolution in application layer.

BlockContent provider_id field removed (meaningless when querying library).

crates/mcp/src/server.rs

@@ -171,7 +171,7 @@ impl KTvMcpServer {
     }
 
     #[tool(
-        description = "Search media items. content_type: movie|episode|short. Returns JSON array of LibraryItem."
+        description = "Search media items. content_type: movie|episode|short. Returns JSON array of MediaItem."
     )]
     async fn search_media(&self, #[tool(aggr)] p: SearchMediaParams) -> String {
         library::search_media(