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 ✅

nginx/conf.d/default.conf

@@ -2,10 +2,9 @@ upstream frontend {
     server frontend:5173;
 }
 
-# Backend będzie dodany później
-# upstream backend {
-#     server backend:3000;
-# }
+upstream backend {
+    server backend:3000;
+}
 
 server {
     listen 80;
@@ -34,16 +33,21 @@ server {
         proxy_read_timeout 60s;
     }
 
-    # Backend API (do dodania później)
-    # location /api {
-    #     proxy_pass http://backend;
-    #     proxy_http_version 1.1;
-    #
-    #     proxy_set_header Host $host;
-    #     proxy_set_header X-Real-IP $remote_addr;
-    #     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-    #     proxy_set_header X-Forwarded-Proto $scheme;
-    # }
+    # Backend API
+    location /api {
+        proxy_pass http://backend;
+        proxy_http_version 1.1;
+
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+
+        # Timeouts
+        proxy_connect_timeout 60s;
+        proxy_send_timeout 60s;
+        proxy_read_timeout 60s;
+    }
 
     # WebSocket dla Socket.IO (do dodania później)
     # location /socket.io {