Adds backend/engine/runner.py with run_single() that iterates pipeline stages,
renders Jinja2 prompt templates with stage history context, checks/stores response
cache, calls LLM adapters, runs configured scorers, creates StageResult and Score
records, and publishes progress events via Redis pub/sub or in-process EventBus.
Includes 21 passing tests covering all execution paths.
Three-stage Dockerfile: frontend-build (Node 20), api (Python 3.12 + uvicorn),
web (nginx 1.27). nginx.conf proxies /api and /ws to the API service with
WebSocket upgrade support. Includes backend/requirements.txt with all Python
deps, frontend scaffolding (Vite + React + TypeScript + Tailwind), and
placeholder alembic files for Docker COPY compatibility.