Migrate database from SQLite to PostgreSQL

- Replace SQLite with PostgreSQL using Neon as provider
- Add environment-based database URL configuration
- Create separate test database setup with DATABASE_URL_TEST
- Reset migration history and generate fresh PostgreSQL schema
- Add comprehensive migration documentation
- Include database unit tests for connection validation

tests/setup.ts

@@ -13,6 +13,11 @@ global.console = {
 process.env.NEXTAUTH_SECRET = 'test-secret';
 process.env.NEXTAUTH_URL = 'http://localhost:3000';
 
+// Use test database for all database operations during tests
+if (process.env.DATABASE_URL_TEST) {
+  process.env.DATABASE_URL = process.env.DATABASE_URL_TEST;
+}
+
 // Mock node-fetch for transcript fetcher tests
 vi.mock('node-fetch', () => ({
   default: vi.fn(),