feat(common): add SongService application layer

crates/common/src/lib.rs

@@ -0,0 +1,28 @@
+use domain::{RepositoryError, Song, SongRepositoryPort, SongSummary, StoredSong};
+use uuid::Uuid;
+
+pub struct SongService {
+    repo: Box<dyn SongRepositoryPort>,
+}
+
+impl SongService {
+    pub fn new(repo: Box<dyn SongRepositoryPort>) -> Self {
+        Self { repo }
+    }
+
+    pub async fn save(&self, song: &Song) -> Result<StoredSong, RepositoryError> {
+        self.repo.save(song).await
+    }
+
+    pub async fn list(&self) -> Result<Vec<SongSummary>, RepositoryError> {
+        self.repo.list().await
+    }
+
+    pub async fn get(&self, id: Uuid) -> Result<Option<Song>, RepositoryError> {
+        self.repo.get(id).await
+    }
+
+    pub async fn delete(&self, id: Uuid) -> Result<(), RepositoryError> {
+        self.repo.delete(id).await
+    }
+}