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/application/src/library/search.rs

@@ -1,5 +1,5 @@
 use domain::DomainResult;
-use domain::models::LibraryItem;
+use domain::models::MediaItem;
 use domain::value_objects::LibrarySearchFilter;
 
 use super::deps::LibraryQueryDeps;
@@ -9,7 +9,7 @@ use super::queries::SearchItemsQuery;
 pub async fn execute(
     deps: &LibraryQueryDeps,
     query: SearchItemsQuery,
-) -> DomainResult<(Vec<LibraryItem>, u32)> {
+) -> DomainResult<(Vec<MediaItem>, u32)> {
     let content_type = query
         .content_type
         .as_deref()