feat(db): drizzle schema, migrations, and seed

packages/backend/drizzle.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'drizzle-kit';
+
+export default defineConfig({
+  schema: './src/db/schema.ts',
+  out: './drizzle',
+  dialect: 'sqlite',
+  dbCredentials: {
+    url: process.env.DB_PATH ?? '../../data/flashcard.db',
+  },
+});