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/fixtures/orders.ts

@@ -0,0 +1,53 @@
+import { mockUser } from './users'
+
+export const mockOrder = {
+  id: 'order-1',
+  userId: mockUser.id,
+  status: 'PENDING' as const,
+  grandTotal: 2999,
+  currency: 'EUR',
+  createdAt: new Date('2024-01-01'),
+  updatedAt: new Date('2024-01-01'),
+  user: mockUser,
+}
+
+export const mockPayment = {
+  id: 'payment-1',
+  orderId: mockOrder.id,
+  provider: 'stripe',
+  providerPaymentId: 'pi_test_123',
+  status: 'pending',
+  amount: 2999,
+  currency: 'EUR',
+  rawPayload: {},
+  createdAt: new Date('2024-01-01'),
+  updatedAt: new Date('2024-01-01'),
+}
+
+export const mockStripeCheckoutEvent = {
+  type: 'checkout.session.completed',
+  data: {
+    object: {
+      metadata: { orderId: mockOrder.id },
+      payment_intent: 'pi_test_123',
+    },
+  },
+}
+
+export const mockStripePaymentSucceededEvent = {
+  type: 'payment_intent.succeeded',
+  data: {
+    object: {
+      id: 'pi_test_123',
+    },
+  },
+}
+
+export const mockStripePaymentFailedEvent = {
+  type: 'payment_intent.payment_failed',
+  data: {
+    object: {
+      id: 'pi_test_123',
+    },
+  },
+}

test/fixtures/users.ts

@@ -0,0 +1,27 @@
+export const mockUser = {
+  id: 'user-1',
+  email: 'test@example.com',
+  name: 'Test User',
+  passwordHash: '$2a$12$hashedpassword',
+  role: 'CUSTOMER' as const,
+  mustChangePassword: false,
+  createdAt: new Date('2024-01-01'),
+  updatedAt: new Date('2024-01-01'),
+}
+
+export const mockAdmin = {
+  ...mockUser,
+  id: 'admin-1',
+  email: 'admin@example.com',
+  name: 'Admin User',
+  role: 'ADMIN' as const,
+}
+
+export const mockSession = {
+  id: 'session-1',
+  userId: mockUser.id,
+  tokenHash: 'abc123hash',
+  expiresAt: new Date(Date.now() + 30 * 24 * 60 * 60 * 1000),
+  createdAt: new Date('2024-01-01'),
+  user: mockUser,
+}