WP-01: domain contract and failing tests for auth and user management

Domain entities/ports, service stubs, 18 application unit tests with in-memory
fakes, API integration tests for /api/auth and /api/users, Vitest specs for the
auth store, login page and user form, Playwright auth/user-management flow.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/domain/src/lib.rs

@@ -1 +1,8 @@
-//! domain layer
+//! Domain layer: entities, value objects, errors and the ports (traits) the application
+//! layer depends on. No I/O here.
+pub mod auth;
+pub mod error;
+pub mod ports;
+pub mod user;
+
+pub use error::DomainError;