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/src/server.js

@@ -0,0 +1,31 @@
+require('dotenv').config();
+const app = require('./app');
+
+const PORT = process.env.PORT || 3000;
+
+const server = app.listen(PORT, '0.0.0.0', () => {
+  console.log('=================================');
+  console.log('🚀 spotlight.cam Backend Started');
+  console.log('=================================');
+  console.log(`Environment: ${process.env.NODE_ENV || 'development'}`);
+  console.log(`Server running on port: ${PORT}`);
+  console.log(`Health check: http://localhost:${PORT}/api/health`);
+  console.log('=================================');
+});
+
+// Graceful shutdown
+process.on('SIGTERM', () => {
+  console.log('SIGTERM received, shutting down gracefully...');
+  server.close(() => {
+    console.log('Server closed');
+    process.exit(0);
+  });
+});
+
+process.on('SIGINT', () => {
+  console.log('SIGINT received, shutting down gracefully...');
+  server.close(() => {
+    console.log('Server closed');
+    process.exit(0);
+  });
+});