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

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

lib/__tests__/product-access.test.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { expandProductAccess, MUSIC_PRODUCT_CHAIN } from '@/lib/product-access';
+
+describe('expandProductAccess', () => {
+  it('일반 제품은 자기 자신만 반환', () => {
+    expect(expandProductAccess(['lotto_tool'])).toEqual(['lotto_tool']);
+  });
+  it('music_pro는 starter를 포함', () => {
+    expect(expandProductAccess(['music_pro']).sort()).toEqual(['music_pro', 'music_starter'].sort());
+  });
+  it('music_master는 전 tier 포함', () => {
+    expect(expandProductAccess(['music_master']).sort()).toEqual(
+      ['music_master', 'music_pro', 'music_starter'].sort(),
+    );
+  });
+  it('중복 입력은 중복 없이 반환', () => {
+    expect(expandProductAccess(['music_pro', 'music_starter']).sort()).toEqual(
+      ['music_pro', 'music_starter'].sort(),
+    );
+  });
+  it('빈 입력은 빈 배열', () => {
+    expect(expandProductAccess([])).toEqual([]);
+  });
+});