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/src/lib.rs

@@ -1,2 +1,8 @@
+pub mod identity;
+pub mod organization;
+pub mod storage;
+pub mod catalog;
+pub mod sharing;
+pub mod sidecar;
+pub mod processing;
 pub mod testing;
-pub mod use_cases;