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/adapters/local-files/src/provider.rs

@@ -4,7 +4,7 @@ use async_trait::async_trait;
 use domain::ports::{
     Collection, IMediaProvider, ProviderCapabilities, StreamQuality, StreamingProtocol,
 };
-use domain::{ContentType, DomainError, DomainResult, MediaFilter, MediaItem, MediaItemId, MediaItemRow};
+use domain::{ContentType, DomainError, DomainResult, MediaFilter, MediaItem, MediaItemId, MediaItemRow, MediaRole};
 
 use crate::config::LocalFilesConfig;
 use crate::index::{decode_id, LocalIndex};
@@ -54,6 +54,12 @@ fn to_media_item(id: MediaItemId, item: &LocalFileItem) -> MediaItem {
         episode_number: None,
         thumbnail_url: None,
         collection_id: None,
+        provider_id: String::new(),
+        external_id: String::new(),
+        collection_name: None,
+        collection_type: None,
+        synced_at: None,
+        role: MediaRole::default(),
     })
 }