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/channel.rs

@@ -342,7 +342,6 @@ impl ProgrammingBlock {
             content: BlockContent::Algorithmic {
                 filter,
                 strategy,
-                provider_id: String::new(),
             },
             loop_on_finish: true,
             ignore_rotation_policy: false,
@@ -362,7 +361,6 @@ impl ProgrammingBlock {
             duration_mins,
             content: BlockContent::Manual {
                 items,
-                provider_id: String::new(),
             },
             loop_on_finish: true,
             ignore_rotation_policy: false,
@@ -403,14 +401,10 @@ impl ProgrammingBlock {
 pub enum BlockContent {
     Manual {
         items: Vec<MediaItemId>,
-        #[serde(default)]
-        provider_id: String,
     },
     Algorithmic {
         filter: MediaFilter,
         strategy: FillStrategy,
-        #[serde(default)]
-        provider_id: String,
     },
 }