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 ✅

backend/.env.example

@@ -0,0 +1,16 @@
+# Server
+NODE_ENV=development
+PORT=3000
+
+# Database (future)
+# DATABASE_URL=postgresql://user:password@db:5432/spotlightcam
+
+# JWT (future)
+# JWT_SECRET=your-secret-key-here
+# JWT_EXPIRES_IN=24h
+
+# CORS
+CORS_ORIGIN=http://localhost:8080
+
+# WebRTC (future)
+# STUN_SERVER=stun:stun.l.google.com:19302