feat: separate Public IP for DNS records from Server IP, improve settings help texts

- Add public_ip field to Settings for DNS A records (proxy/load balancer IP)
- DNS records now use public_ip, falling back to server_ip if empty
- Server IP renamed to "Server IP (Docker Host)" for clarity
- Public IP labeled "Public IP (DNS Target)"
- Updated help texts for domain, server IP, public IP, and Docker network
- DB migration + schema for public_ip column

internal/store/models.go

@@ -49,7 +49,8 @@ type Registry struct {
 // Settings holds global application configuration (single-row pattern).
 type Settings struct {
 	Domain           string `json:"domain"`
-	ServerIP         string `json:"server_ip"`
+	ServerIP         string `json:"server_ip"`  // Docker host IP (for NPM remote forwarding)
+	PublicIP         string `json:"public_ip"`  // Public-facing IP for DNS A records (e.g., NPM/proxy host)
 	Network          string `json:"network"`
 	SubdomainPattern string `json:"subdomain_pattern"`
 	NotificationURL  string `json:"notification_url"`