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/app.js

@@ -0,0 +1,54 @@
+const express = require('express');
+const cors = require('cors');
+
+const app = express();
+
+// Middleware
+app.use(cors({
+  origin: process.env.CORS_ORIGIN || 'http://localhost:8080',
+  credentials: true
+}));
+app.use(express.json());
+app.use(express.urlencoded({ extended: true }));
+
+// Request logging middleware
+app.use((req, res, next) => {
+  console.log(`${new Date().toISOString()} - ${req.method} ${req.path}`);
+  next();
+});
+
+// Health check endpoint
+app.get('/api/health', (req, res) => {
+  res.status(200).json({
+    status: 'ok',
+    message: 'Backend is running',
+    timestamp: new Date().toISOString(),
+    environment: process.env.NODE_ENV || 'development'
+  });
+});
+
+// API routes (future)
+// app.use('/api/auth', require('./routes/auth'));
+// app.use('/api/users', require('./routes/users'));
+// app.use('/api/events', require('./routes/events'));
+// app.use('/api/matches', require('./routes/matches'));
+// app.use('/api/ratings', require('./routes/ratings'));
+
+// 404 handler
+app.use((req, res) => {
+  res.status(404).json({
+    error: 'Not Found',
+    message: `Cannot ${req.method} ${req.path}`
+  });
+});
+
+// Error handler
+app.use((err, req, res, next) => {
+  console.error('Error:', err);
+  res.status(err.status || 500).json({
+    error: err.message || 'Internal Server Error',
+    ...(process.env.NODE_ENV === 'development' && { stack: err.stack })
+  });
+});
+
+module.exports = app;