Add Docker Compose for one-command local setup

[denial-web]

What

Create a docker-compose.yml that spins up the entire stack (backend, frontend, Redis, database) with one command.

Why

New users should be able to run docker compose up and have a working system in under 2 minutes. This dramatically lowers the barrier to trying the project.

Scope

• Create docker-compose.yml with services: backend, frontend, redis
• Create Dockerfile for the backend
• Update README with Docker quick start option
• Ensure npm run demo-escalation works against the containerized stack

Current setup

See docs/SETUP.md for the manual setup process.