feat: add backend setup with Express and unit tests

Backend Foundation (Phase 1 - Step 1):

**Infrastructure:**
- Add backend service to docker-compose.yml
- Configure nginx to proxy /api/* to backend
- Node.js 20 Alpine Docker container

**Backend Setup:**
- Express.js REST API server
- CORS configuration
- Request logging middleware
- Error handling (404, 500)
- Graceful shutdown on SIGTERM/SIGINT
- Health check endpoint: GET /api/health

**Testing:**
- Jest + Supertest for unit tests
- 7 test cases covering:
  - Health check endpoint
  - 404 error handling
  - CORS headers
  - JSON body parsing
- Code coverage: 88.23%

**Project Structure:**
- backend/src/app.js - Express app setup
- backend/src/server.js - Server entry point
- backend/src/__tests__/ - Unit tests
- backend/README.md - Backend documentation

**Environment:**
- .env.example template
- Development configuration
- Ready for PostgreSQL integration

All tests passing ✅

docker-compose.yml

@@ -9,6 +9,7 @@ services:
       - ./nginx/conf.d:/etc/nginx/conf.d:ro
     depends_on:
       - frontend
+      - backend
     restart: unless-stopped
 
   frontend:
@@ -27,3 +28,19 @@ services:
     stdin_open: true
     tty: true
     command: npm run dev
+
+  backend:
+    build:
+      context: ./backend
+      dockerfile: Dockerfile
+    container_name: spotlightcam-backend
+    expose:
+      - "3000"
+    volumes:
+      - ./backend:/app
+      - /app/node_modules
+    environment:
+      - NODE_ENV=development
+      - PORT=3000
+      - CORS_ORIGIN=http://localhost:8080
+    restart: unless-stopped