feat(products): vitest 도입 + 제품 접근 확장 로직 (music tier 하위 호환)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+import path from 'path';
+
+export default defineConfig({
+  test: { include: ['lib/**/*.test.ts'] },
+  resolve: { alias: { '@': path.resolve(__dirname, '.') } },
+});