app: add core use cases (RegisterUser, CreateAlbum) with TDD

crates/application/tests/use_cases/identity/mod.rs

@@ -0,0 +1 @@
+mod register_user;

crates/application/tests/use_cases/identity/register_user.rs

@@ -0,0 +1,48 @@
+use std::sync::Arc;
+use application::testing::{InMemoryUserRepository, StubPasswordHasher};
+use application::use_cases::RegisterUser;
+use domain::errors::DomainError;
+
+#[tokio::test]
+async fn registers_new_user() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let uc = RegisterUser::new(repo.clone(), Arc::new(StubPasswordHasher));
+    let user = uc.execute("testuser", "test@example.com", "password123").await.unwrap();
+    assert_eq!(user.username, "testuser");
+    assert_eq!(user.email.as_str(), "test@example.com");
+    assert_eq!(repo.all().await.len(), 1);
+}
+
+#[tokio::test]
+async fn rejects_duplicate_email() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let uc = RegisterUser::new(repo.clone(), Arc::new(StubPasswordHasher));
+    uc.execute("user1", "test@example.com", "password123").await.unwrap();
+    let result = uc.execute("user2", "test@example.com", "different1").await;
+    assert!(matches!(result, Err(DomainError::Conflict(_))));
+}
+
+#[tokio::test]
+async fn rejects_duplicate_username() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let uc = RegisterUser::new(repo.clone(), Arc::new(StubPasswordHasher));
+    uc.execute("sameuser", "a@example.com", "password123").await.unwrap();
+    let result = uc.execute("sameuser", "b@example.com", "password123").await;
+    assert!(matches!(result, Err(DomainError::Conflict(_))));
+}
+
+#[tokio::test]
+async fn rejects_short_password() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let uc = RegisterUser::new(repo, Arc::new(StubPasswordHasher));
+    let result = uc.execute("user", "test@example.com", "short").await;
+    assert!(matches!(result, Err(DomainError::Validation(_))));
+}
+
+#[tokio::test]
+async fn rejects_empty_username() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let uc = RegisterUser::new(repo, Arc::new(StubPasswordHasher));
+    let result = uc.execute("", "test@example.com", "password123").await;
+    assert!(matches!(result, Err(DomainError::Validation(_))));
+}