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

ts_interfaces/requests/email-domains.ts

@@ -55,6 +55,8 @@ export interface IReq_CreateEmailDomain extends plugins.typedrequestInterfaces.i
     apiToken?: string;
     /** ID of the existing dcrouter DNS domain to link to. */
     linkedDomainId: string;
+    /** Optional subdomain prefix (e.g. 'mail' for mail.example.com). Leave empty for bare domain. */
+    subdomain?: string;
     /** DKIM selector (default: 'default'). */
     dkimSelector?: string;
     /** RSA key size (default: 2048). */