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>

frontend/e2e/smoke.spec.ts

@@ -1,7 +1,7 @@
 import { test, expect } from '@playwright/test'
 
-test('app loads and shows backend health', async ({ page }) => {
-  await page.goto('/')
+test('login page loads', async ({ page }) => {
+  await page.goto('/login')
   await expect(page.getByText('SoftVisor Monitoring')).toBeVisible()
-  await expect(page.getByTestId('backend-status')).toContainText('ok')
+  await expect(page.getByRole('button', { name: 'Sign in' })).toBeVisible()
 })