Add Docker Compose setup

Multi-stage Dockerfile builds the React frontend then serves everything
from a single Python container. settings.json persists via a named volume.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,17 @@
+FROM node:22-alpine AS build
+WORKDIR /app/frontend
+COPY frontend/package*.json ./
+RUN npm ci
+COPY frontend/ ./
+RUN npm run build
+
+FROM python:3.12-slim
+WORKDIR /app
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+COPY backend/ ./backend/
+COPY --from=build /app/frontend/dist ./frontend/dist
+VOLUME ["/app/data"]
+ENV SETTINGS_PATH=/app/data/settings.json
+EXPOSE 8000
+CMD ["uvicorn", "backend.main:app", "--host", "0.0.0.0", "--port", "8000"]