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/api/settings.go

@@ -21,6 +21,7 @@ import (
 type settingsRequest struct {
 	Domain           string `json:"domain"`
 	ServerIP         string `json:"server_ip"`
+	PublicIP         string `json:"public_ip"`
 	Network          string `json:"network"`
 	SubdomainPattern string `json:"subdomain_pattern"`
 	NotificationURL  string `json:"notification_url"`
@@ -59,6 +60,7 @@ func (s *Server) getSettings(w http.ResponseWriter, r *http.Request) {
 	respondJSON(w, http.StatusOK, map[string]any{
 		"domain":                    settings.Domain,
 		"server_ip":                 settings.ServerIP,
+		"public_ip":                 settings.PublicIP,
 		"network":                   settings.Network,
 		"subdomain_pattern":         settings.SubdomainPattern,
 		"notification_url":          settings.NotificationURL,
@@ -107,6 +109,9 @@ func (s *Server) updateSettings(w http.ResponseWriter, r *http.Request) {
 	if req.ServerIP != "" {
 		updated.ServerIP = req.ServerIP
 	}
+	if req.PublicIP != "" {
+		updated.PublicIP = req.PublicIP
+	}
 	if req.Network != "" {
 		updated.Network = req.Network
 	}