feat(email-domains): support creating email domains on optional subdomains

ts_interfaces/data/email-domain.ts

@@ -12,10 +12,12 @@ export type TDnsRecordStatus = 'valid' | 'missing' | 'invalid' | 'unchecked';
  */
 export interface IEmailDomain {
   id: string;
-  /** Fully qualified domain name (e.g. 'example.com'). */
+  /** Fully qualified email domain name (e.g. 'example.com' or 'mail.example.com'). */
   domain: string;
   /** ID of the linked dcrouter DNS domain — determines how DNS records are managed. */
   linkedDomainId: string;
+  /** Optional subdomain prefix (e.g. 'mail' for mail.example.com). Empty/undefined = bare domain. */
+  subdomain?: string;
   /** DKIM configuration and key state. */
   dkim: IEmailDomainDkim;
   /** Optional per-domain rate limits. */