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/application/src/lib.rs

@@ -1 +1,11 @@
-//! application layer
+//! Application layer: use cases orchestrating the domain through its ports.
+pub mod auth_service;
+pub mod user_service;
+
+pub use auth_service::AuthService;
+pub use user_service::UserService;
+
+#[cfg(test)]
+pub(crate) mod test_fakes;
+#[cfg(test)]
+mod tests;