test: add Vitest infrastructure (config, setup, mocks, fixtures)

Adds test harness for the suite:
- vitest.config.ts: happy-dom env, @/* alias, v8 coverage with 70% thresholds
- setup.ts: env vars, global next/headers and next/navigation mocks
- tsconfig.json: IDE alias resolution for test/
- __mocks__/prisma.ts: centralised Prisma mock auto-registered via vi.mock
- fixtures/users.ts, fixtures/orders.ts: typed test data

test/setup.ts

@@ -0,0 +1,31 @@
+import '@testing-library/jest-dom'
+import { vi } from 'vitest'
+
+// Env vars necessarie per i test
+process.env.STRIPE_SECRET_KEY = 'sk_test_mock'
+process.env.STRIPE_WEBHOOK_SECRET = 'whsec_test_mock'
+process.env.SMTP_HOST = 'localhost'
+process.env.SMTP_PORT = '1025'
+process.env.APP_URL = 'http://localhost'
+process.env.DATABASE_URL = 'postgresql://test:test@localhost:5432/test'
+process.env.NODE_ENV = 'test'
+process.env.AUTH_SECRET = 'test-secret-32-chars-minimum-ok!'
+
+// Mock di next/headers (cookies() lancia fuori dal runtime Next.js)
+vi.mock('next/headers', () => {
+  const mockCookieStore = {
+    get: vi.fn(),
+    set: vi.fn(),
+    delete: vi.fn(),
+  }
+  return {
+    cookies: vi.fn(() => mockCookieStore),
+  }
+})
+
+// Mock di next/navigation
+vi.mock('next/navigation', () => ({
+  useRouter: vi.fn(() => ({ push: vi.fn(), replace: vi.fn() })),
+  usePathname: vi.fn(() => '/'),
+  redirect: vi.fn(),
+}))