Add Node.js/Express API service for booking backend

- New api/ service: Express + better-sqlite3, health check at GET /api/health,
  POST /api/bookings stores sessions with all required fields
- Dockerfile with Alpine build deps for native sqlite3 module
- docker-compose.yml: api service with volume mounts for data, signed-contracts, contracts
- nginx.conf: proxy /api/ to api:3001; Immich regex locations remain higher priority
- .gitignore: exclude SQLite db file and signed PDFs from version control

Closes #1

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

api/Dockerfile

@@ -0,0 +1,16 @@
+FROM node:20-alpine
+
+RUN apk add --no-cache python3 make g++
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci --only=production
+
+COPY . .
+
+RUN mkdir -p data signed-contracts contracts
+
+EXPOSE 3001
+
+CMD ["node", "server.js"]