fix: update schema to fix multiple primary key issues and switch to node-postgres

- Replace primaryKey constraints with uniqueIndex where needed
- Fix schema for Auth, Contacts, Exchange Rates, Expense Participants, and Accounts tables
- Add missing import for uniqueIndex
- Switch from postgres.js to node-postgres (pg) for Drizzle
- Add proper type safety with TypeScript
- Add graceful shutdown of database connections

backend/.env.example

@@ -0,0 +1,5 @@
+PORT=3000
+DATABASE_URL=postgres://postgres:postgres@localhost:5432/splitwise
+JWT_SECRET=your-jwt-secret-key-here
+NODE_ENV=development
+DATABASE_SSL=false