test: Phase 0 测试基础设施 — TestApp + MSW + 覆盖率工具 + CI

- TestApp struct 封装 TestDb + HealthState + tenant_id/operator_id
- TestFixture 工厂方法: create_patient/create_doctor/create_schedule/create_appointment
- 前端 MSW v2 handlers (auth) + server setup + vitest 集成
- vitest coverage v8 配置 + test:coverage script
- GitHub Actions CI: backend (check + test + clippy) + frontend (tsc + test + build)

crates/erp-server/tests/integration.rs

@@ -1,5 +1,7 @@
 #[path = "integration/test_db.rs"]
 mod test_db;
+#[path = "integration/test_fixture.rs"]
+mod test_fixture;
 #[path = "integration/auth_tests.rs"]
 mod auth_tests;
 #[path = "integration/plugin_tests.rs"]