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

Cargo.lock

@@ -92,6 +92,7 @@ dependencies = [
  "async-trait",
  "bytes",
  "domain",
+ "serde",
  "thiserror",
  "tokio",
  "uuid",