feat: Implement the core InterNetX DDNS updater application with API, services, middleware, Docker support, and a detailed README.

.env.example

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+# Server Configuration
+PORT=3000
+NODE_ENV=production
+
+# InterNetX API Configuration
+# Get your credentials from: https://www.internetx.com/
+INTERNETX_API_URL=https://api.autodns.com/v1
+INTERNETX_USER=your-username
+INTERNETX_PASSWORD=your-password
+INTERNETX_CONTEXT=4
+
+# DDNS Configuration
+# The domain zone managed by this service (e.g., ddns.netstack.berlin)
+DEFAULT_ZONE=ddns.netstack.berlin
+
+# Default TTL for DNS records (in seconds)
+DEFAULT_TTL=300
+
+# Security Configuration
+# Generate secure tokens with: node -e "console.log(require('crypto').randomBytes(64).toString('hex'))"
+# Comma-separated list of valid authentication tokens
+AUTH_TOKENS=your-token-here,another-token-here
+
+# Optional: IP Whitelist (comma-separated CIDR ranges)
+# Leave empty to allow all IPs
+# Example: IP_WHITELIST=192.168.1.0/24,10.0.0.0/8
+IP_WHITELIST=
+
+# Rate Limiting
+# Time window in milliseconds
+RATE_LIMIT_WINDOW_MS=300000
+
+# Maximum requests per IP within the time window
+RATE_LIMIT_MAX_REQUESTS=20
+
+# Logging Configuration
+LOG_LEVEL=info
+LOG_FILE_MAX_SIZE=10m
+LOG_FILE_MAX_AGE=30d