fix(admin): preserve revoked owners and 503 status

Owner recovery lived in the identity-only migration path and could clear historical revocations on conflict. Admin layout also rethrew authorization 503s, turning service-unavailable failures into 500 responses.\n\nMove recovery behind the RBAC migration sequence, exclude revoked candidates without mutating their history, and terminate layout redirects at a no-store 503 route.

frontend/src/app/admin-unavailable/route.ts

@@ -0,0 +1,9 @@
+export function GET() {
+  return new Response("后台服务暂时不可用", {
+    status: 503,
+    headers: {
+      "cache-control": "no-store",
+      "content-type": "text/plain; charset=utf-8",
+    },
+  });
+}

frontend/src/app/admin/layout.tsx

@@ -17,6 +17,7 @@ export default async function AdminLayout({ children }: { children: ReactNode })
       const failure = resolveAdminPageAccessFailure(error.status);
       if (failure.kind === "login") redirect(failure.location);
       if (failure.kind === "forbidden") forbidden();
+      if (failure.kind === "unavailable") redirect(failure.location);
     }
     throw error;
   }