feat: track oauth new-user result

backend/src/main/java/com/openisle/model/InviteToken.java

@@ -0,0 +1,23 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Data;
+
+import java.time.LocalDate;
+
+/**
+ * Invite token entity tracking usage counts.
+ */
+@Data
+@Entity
+public class InviteToken {
+    @Id
+    private String token;
+
+    @ManyToOne
+    private User inviter;
+
+    private LocalDate createdDate;
+
+    private int usageCount;
+}