Migrate tests from Jest to Vitest, updating setup and test files accordingly.

- Replace Jest imports and mocks with Vitest equivalents in setup and unit tests.
- Adjust test cases to use async imports and reset modules with Vitest.
- Add Vitest configuration file for test environment setup and coverage reporting.

tests/setup.ts

@@ -1,25 +1,19 @@
-// Jest test setup
-import { jest } from '@jest/globals';
+// Vitest test setup
+import { vi } from 'vitest';
 
 // Mock console methods to reduce noise in tests
 global.console = {
   ...console,
-  log: jest.fn(),
-  warn: jest.fn(),
-  error: jest.fn(),
+  log: vi.fn(),
+  warn: vi.fn(),
+  error: vi.fn(),
 };
 
 // Set test environment variables
-Object.defineProperty(process.env, 'NODE_ENV', {
-  value: 'test',
-  writable: true,
-  configurable: true,
-});
 process.env.NEXTAUTH_SECRET = 'test-secret';
 process.env.NEXTAUTH_URL = 'http://localhost:3000';
 
 // Mock node-fetch for transcript fetcher tests
-jest.mock('node-fetch', () => ({
-  __esModule: true,
-  default: jest.fn(),
+vi.mock('node-fetch', () => ({
+  default: vi.fn(),
 }));