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/store.go

@@ -119,6 +119,8 @@ func (s *Store) runMigrations() error {
 		// NPM access list support (global default + per-project override).
 		`ALTER TABLE settings ADD COLUMN npm_access_list_id INTEGER NOT NULL DEFAULT 0`,
 		`ALTER TABLE projects ADD COLUMN npm_access_list_id INTEGER NOT NULL DEFAULT 0`,
+		// Separate public IP for DNS A records.
+		`ALTER TABLE settings ADD COLUMN public_ip TEXT NOT NULL DEFAULT ''`,
 	}
 
 	for _, m := range migrations {
@@ -211,6 +213,7 @@ CREATE TABLE IF NOT EXISTS settings (
 	id                  INTEGER PRIMARY KEY CHECK (id = 1),
 	domain              TEXT NOT NULL DEFAULT '',
 	server_ip           TEXT NOT NULL DEFAULT '',
+	public_ip           TEXT NOT NULL DEFAULT '',
 	network             TEXT NOT NULL DEFAULT 'docker-watcher',
 	subdomain_pattern   TEXT NOT NULL DEFAULT 'stage-{stage}-{project}',
 	notification_url    TEXT NOT NULL DEFAULT '',