Add composite key for user role to prevent duplication.

Author: vytaux

src/main/java/com/theateam/waaprojectspringbackend/entity/User.java

@@ -9,7 +9,9 @@
 
 @Entity
 @Data
-@Table(name = "users")
+@Table(name = "users", uniqueConstraints = {
+    @UniqueConstraint(columnNames = {"user_id", "role_id"})
+})
 public class User {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)