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/domain/src/models/tests/channel.rs

@@ -102,9 +102,8 @@ fn manual_block_creation() {
     let items = vec![MediaItemId::new("item1"), MediaItemId::new("item2")];
     let block = ProgrammingBlock::new_manual("Manual Block", t(20, 0), 60, items);
     match block.content() {
-        BlockContent::Manual { items, provider_id } => {
+        BlockContent::Manual { items } => {
             assert_eq!(items.len(), 2);
-            assert!(provider_id.is_empty());
         }
         _ => panic!("Expected Manual content"),
     }