feat: add comprehensive unit and integration testing infrastructure

- Backend: pytest with unit tests (auth, rbac, config) and integration tests (auth flow, docker, files)
- Frontend: vitest with unit tests (api client) and component tests (login)
- CI: Gitea Actions workflow for automated testing with coverage reports
- Documentation: TESTING.md guide with setup, usage, and best practices
- Coverage goals: 80%+ backend, 70%+ frontend

dashboard/backend/requirements-dev.txt

@@ -0,0 +1,4 @@
+pytest==8.3.4
+pytest-asyncio==0.24.0
+pytest-cov==6.0.0
+pytest-mock==3.14.0