feat: add Drizzle ORM support and update README with environment setup and migration scripts

Backend/db/client.ts

@@ -0,0 +1,15 @@
+import 'dotenv/config';
+import { drizzle } from 'drizzle-orm/node-postgres';
+import { Pool } from 'pg';
+
+import * as schema from './schema';
+
+if (!process.env.DATABASE_URL) {
+  throw new Error('DATABASE_URL is not set');
+}
+
+export const pool = new Pool({
+  connectionString: process.env.DATABASE_URL,
+});
+
+export const db = drizzle(pool, { schema });