feat: add comprehensive test suite for OAuth, Backup, and E2E

- Add OAuth unit and integration tests (passport.test.ts, auth.routes.oauth.test.ts)
- Add backup service and routes tests (backup.service.test.ts, backup.routes.test.ts)
- Set up Playwright E2E test framework with auth and recipe tests
- Add email service tests
- Increase test count from 99 to 210+ tests
- Configure Playwright for cross-browser E2E testing
- Add test coverage for critical new features (Google OAuth, Backup/Restore)

Test Coverage:
- OAuth: Comprehensive tests for Google login flow, callbacks, error handling
- Backup: Tests for creation, restoration, validation, error handling
- E2E: Authentication flow, recipe CRUD operations
- Email: SMTP configuration and template tests

This significantly improves code quality and confidence in deployments.

packages/api/playwright.config.ts

@@ -0,0 +1,40 @@
+import { defineConfig, devices } from '@playwright/test';
+
+/**
+ * Playwright E2E Test Configuration
+ * See https://playwright.dev/docs/test-configuration
+ */
+export default defineConfig({
+  testDir: './e2e',
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: [
+    ['html'],
+    ['list'],
+    ['json', { outputFile: 'test-results/e2e-results.json' }],
+  ],
+  use: {
+    baseURL: process.env.E2E_BASE_URL || 'http://localhost:5173',
+    trace: 'on-first-retry',
+    screenshot: 'only-on-failure',
+    video: 'retain-on-failure',
+  },
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+    },
+    {
+      name: 'firefox',
+      use: { ...devices['Desktop Firefox'] },
+    },
+  ],
+  webServer: {
+    command: 'npm run dev',
+    url: 'http://localhost:5173',
+    reuseExistingServer: !process.env.CI,
+    timeout: 120 * 1000,
+  },
+});