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/main.rs

@@ -48,7 +48,7 @@ async fn main() -> anyhow::Result<()> {
     let event_publisher: Arc<dyn domain::ports::EventPublisher> = event_bus.clone();
 
     let schedule_engine = Arc::new(ScheduleEngineService::new(
-        provider_registry.clone(),
+        wire.library_query.clone(),
         wire.channel_query.clone(),
         wire.schedule_query.clone(),
         wire.schedule_command.clone(),
@@ -70,6 +70,7 @@ async fn main() -> anyhow::Result<()> {
         schedule_query: wire.schedule_query.clone(),
         schedule_command: wire.schedule_command.clone(),
         event_publisher: event_publisher.clone(),
+        provider_registry: provider_registry.clone(),
     });
 
     let library_query_deps = Arc::new(application::library::LibraryQueryDeps {