feat(tests): Jest backend (38 tests), Jest RTL frontend (28 tests), Playwright E2E spec

backend/tests/setup.js

@@ -0,0 +1,6 @@
+// Global setup — runs once before all test suites.
+// When DATABASE_URL points to a real test DB, Prisma migrations are applied here.
+// In CI/local runs without a live DB, tests use mocked Prisma via jest.mock.
+module.exports = async function () {
+  process.env.NODE_ENV = 'test';
+};