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/value_objects/scheduling.rs

@@ -91,6 +91,14 @@ impl Weekday {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "snake_case")]
+pub enum MediaRole {
+    #[default]
+    Program,
+    Interstitial,
+}
+
 #[cfg(test)]
 #[path = "tests/scheduling.rs"]
 mod tests;