domain: add Media Catalog entities (Asset, Metadata, Stack, Derivative, Duplicate)

crates/domain/tests/entities/mod.rs

@@ -6,3 +6,8 @@ mod storage_volume;
 mod library_path;
 mod ingest_session;
 mod quota;
+mod asset;
+mod asset_metadata;
+mod asset_stack;
+mod derivative_asset;
+mod duplicate;