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)

apps/web/src/test/mocks/server.ts

@@ -0,0 +1,4 @@
+import { setupServer } from 'msw/node';
+import { handlers } from './handlers';
+
+export const server = setupServer(...handlers);