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>

docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  db:
+    image: postgres:16-alpine
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      POSTGRES_DB: ${POSTGRES_DB}
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+
+  app:
+    build: .
+    restart: unless-stopped
+    depends_on:
+      - db
+    ports:
+      - "3000:3000"
+    environment:
+      DATABASE_URL: ${DATABASE_URL}
+      PORT: ${PORT}
+
+volumes:
+  pgdata: