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/tests/mod.rs

@@ -0,0 +1,2 @@
+mod auth_service_tests;
+mod user_service_tests;