server {
    listen 80;
    server_name _;
    root /usr/share/nginx/html;
    index index.html;

    # DNS resolver for dynamic upstream resolution
    # This allows nginx to start even if backend doesn't exist yet
    resolver 127.0.0.11 valid=30s ipv6=off;

    # Gzip compression
    gzip on;
    gzip_vary on;
    gzip_min_length 1024;
    gzip_types text/plain text/css text/xml text/javascript application/x-javascript application/xml+rss application/json application/javascript;

    # Security headers
    add_header X-Frame-Options "SAMEORIGIN" always;
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-XSS-Protection "1; mode=block" always;

    # Handle API requests - proxy to backend (docker-compose only)
    # Uses variables to force runtime DNS resolution instead of startup
    location /api/ {
        # Use variable to force runtime DNS resolution
        set $backend "backend:8000";
        proxy_pass http://$backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        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;
        proxy_cache_bypass $http_upgrade;

        # Handle backend connection errors gracefully
        proxy_intercept_errors on;
        error_page 502 503 504 = @backend_unavailable;
    }

    # Fallback for when backend is unavailable
    location @backend_unavailable {
        add_header Content-Type application/json;
        return 503 '{"error": "Backend service unavailable. Configure VITE_BACKEND_URL in frontend or ensure backend container is running."}';
    }

    # Serve static files
    location / {
        try_files $uri $uri/ /index.html;
    }

    # Cache static assets
    location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
        expires 1y;
        add_header Cache-Control "public, immutable";
    }
}