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

Backend/drizzle.config.ts

@@ -0,0 +1,17 @@
+import 'dotenv/config';
+import { defineConfig } from 'drizzle-kit';
+
+if (!process.env.DATABASE_URL) {
+  throw new Error('DATABASE_URL is not set');
+}
+
+export default defineConfig({
+  schema: './db/schema.ts',
+  out: './drizzle',
+  dialect: 'postgresql',
+  dbCredentials: {
+    url: process.env.DATABASE_URL,
+  },
+  strict: true,
+  verbose: true,
+});