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/README.md

@@ -0,0 +1,127 @@
+# spotlight.cam Backend
+
+Node.js + Express backend for spotlight.cam - P2P video exchange app for dance events.
+
+## Features
+
+- ✅ Express REST API
+- ✅ CORS enabled
+- ✅ Health check endpoint
+- ✅ Error handling
+- ✅ Unit tests (Jest + Supertest)
+- ⏳ PostgreSQL integration (planned)
+- ⏳ JWT authentication (planned)
+- ⏳ Socket.IO for real-time chat (planned)
+- ⏳ WebRTC signaling (planned)
+
+## API Endpoints
+
+### Health Check
+- `GET /api/health` - Backend health status
+
+### Future Endpoints
+- `POST /api/auth/register` - Register new user
+- `POST /api/auth/login` - Login user
+- `GET /api/users/me` - Get current user
+- `GET /api/events` - List events
+- `POST /api/matches` - Create match
+- `POST /api/ratings` - Rate partner
+
+## Development
+
+### Install dependencies
+```bash
+npm install
+```
+
+### Run in development mode
+```bash
+npm run dev
+```
+
+### Run tests
+```bash
+npm test
+```
+
+### Run tests in watch mode
+```bash
+npm run test:watch
+```
+
+### Run in production mode
+```bash
+npm start
+```
+
+## Environment Variables
+
+Create a `.env` file (see `.env.example`):
+
+```env
+NODE_ENV=development
+PORT=3000
+CORS_ORIGIN=http://localhost:8080
+```
+
+## Project Structure
+
+```
+backend/
+├── src/
+│   ├── __tests__/          # Unit tests
+│   │   └── app.test.js
+│   ├── routes/             # API routes (future)
+│   ├── controllers/        # Business logic (future)
+│   ├── middleware/         # Custom middleware (future)
+│   ├── utils/              # Helper functions (future)
+│   ├── app.js              # Express app setup
+│   └── server.js           # Server entry point
+├── .env                    # Environment variables (gitignored)
+├── .env.example            # Environment variables template
+├── package.json
+└── Dockerfile
+```
+
+## Testing
+
+Tests are written using:
+- **Jest** - Test framework
+- **Supertest** - HTTP assertions
+
+Run tests:
+```bash
+npm test
+```
+
+Current test coverage:
+- Health check endpoint
+- 404 error handling
+- CORS configuration
+- JSON body parsing
+
+## Docker
+
+Build and run with Docker Compose (from project root):
+```bash
+docker compose up --build
+```
+
+Backend will be available at:
+- Internal: http://backend:3000
+- Through nginx: http://localhost:8080/api
+
+## Next Steps
+
+1. ✅ Basic Express setup
+2. ✅ Health check endpoint
+3. ✅ Unit tests
+4. ⏳ PostgreSQL connection
+5. ⏳ Database schema and migrations
+6. ⏳ Authentication (JWT + bcrypt)
+7. ⏳ Socket.IO for real-time chat
+8. ⏳ WebRTC signaling
+
+## License
+
+TBD