Add Docker Compose project scaffold

Sets up the full monorepo structure with:
- Multi-stage Dockerfile (client-build + production stages)
- docker-compose.yml for production, docker-compose.dev.yml overlay for development
- Express server (port 3000) with pg pool, health route, and SPA static file serving
- React 18 + Vite client with react-router-dom v6, nav bar, and placeholder page components
- .env.example with PostgreSQL and app config
- Empty db/migrations/ directory for future migrations
- CLAUDE.md updated with development workflow commands

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

Dockerfile

@@ -0,0 +1,27 @@
+# Stage 1: build the React client
+FROM node:20-alpine AS client-build
+
+WORKDIR /app/client
+
+COPY client/package.json ./
+RUN npm install
+
+COPY client/ ./
+RUN npm run build
+
+# Stage 2: production server
+FROM node:20-alpine AS production
+
+WORKDIR /app/server
+
+COPY server/package.json ./
+RUN npm install --omit=dev
+
+COPY server/ ./
+
+# Copy built client assets
+COPY --from=client-build /app/client/dist /app/client/dist
+
+EXPOSE 3000
+
+CMD ["node", "src/index.js"]