Initial scaffold: tablet-checkin app

Staff-push guest check-in tablet. Fastify backend + React/Vite frontend,
three-tier auth (staff JWT, terminal API key, kiosk cookie), NewBook
integration via bookings_get/guests_update/bookings_update, pdfkit-based
regcard generation, marketing opt-in with Sendy/Mailchimp adapters.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Dockerfile

@@ -0,0 +1,8 @@
+FROM node:22-alpine
+WORKDIR /app
+COPY package.json .
+RUN npm install --omit=dev
+COPY src ./src
+RUN mkdir -p /app/uploads
+EXPOSE 3001
+CMD ["node", "src/index.js"]