refactor: restructure application to CQRS, update api-types + presentation

- application: replace flat use_cases/ with identity/{commands,queries}/ and organization/commands/
- each use case now split into Command/Query struct + Handler struct
- api-types: add username to RegisterRequest/UserResponse, add CreateAlbumRequest/AlbumResponse
- presentation: update state, handlers, factory to use new handler types
- tests: restructured to match CQRS module layout, added get_profile tests

crates/application/tests/identity/queries/get_profile.rs

@@ -0,0 +1,28 @@
+use std::sync::Arc;
+use application::testing::{InMemoryUserRepository, StubPasswordHasher};
+use application::identity::{RegisterUserCommand, RegisterUserHandler, GetProfileQuery, GetProfileHandler};
+use domain::errors::DomainError;
+use domain::value_objects::SystemId;
+
+#[tokio::test]
+async fn returns_existing_user() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let reg = RegisterUserHandler::new(repo.clone(), Arc::new(StubPasswordHasher));
+    let user = reg.execute(RegisterUserCommand {
+        username: "alice".into(),
+        email: "alice@example.com".into(),
+        password: "password123".into(),
+    }).await.unwrap();
+
+    let handler = GetProfileHandler::new(repo);
+    let found = handler.execute(GetProfileQuery { user_id: user.id }).await.unwrap();
+    assert_eq!(found.username, "alice");
+}
+
+#[tokio::test]
+async fn returns_not_found_for_missing_user() {
+    let repo = Arc::new(InMemoryUserRepository::new());
+    let handler = GetProfileHandler::new(repo);
+    let result = handler.execute(GetProfileQuery { user_id: SystemId::new() }).await;
+    assert!(matches!(result, Err(DomainError::NotFound(_))));
+}