fix(domains): backend column showed (undefined:undefined) instead of scheme

BackendLite interfaces in Domains list and Detail declared address/port
fields that don't exist on the Backend model (those live on BackendServer).
Every backend tag rendered as "name (undefined:undefined)". Fixed by
replacing the phantom fields with scheme, so tags show e.g. "my-app (https)".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

management-ui/src/pages/Domains/index.tsx

@@ -62,8 +62,7 @@ async function listDomains(): Promise<Domain[]> {
 interface BackendLite {
   id: number
   name: string
-  address: string
-  port: number
+  scheme: string
   active: boolean
 }
 async function listBackends(): Promise<BackendLite[]> {
@@ -222,7 +221,7 @@ export default function DomainsPage() {
         return (
           <Space size={4}>
             {b
-              ? <Tag color="blue">{b.name} ({b.address}:{b.port})</Tag>
+              ? <Tag color="blue">{b.name} ({b.scheme})</Tag>
               : <Tag color="orange">#{id}</Tag>}
             {health === 'UP'   && <Tag color="green"  style={{ margin: 0 }}>UP</Tag>}
             {health === 'DOWN' && <Tag color="red"    style={{ margin: 0 }}>DOWN</Tag>}