feat: track oauth new-user result

backend/src/main/java/com/openisle/service/AuthResult.java

@@ -0,0 +1,12 @@
+package com.openisle.service;
+
+import com.openisle.model.User;
+import lombok.Value;
+
+/** Result for OAuth authentication indicating whether a new user was created. */
+@Value
+public class AuthResult {
+    User user;
+    boolean newUser;
+}
+