refactor(user): clean up comments and rename variables

backend/src/main/java/de/szut/casino/user/UserEntity.java

@@ -19,7 +19,7 @@ public class UserEntity {
     @GeneratedValue
     private Long id;
     @Column(unique = true)
-    private String authentikId;  // Changed from keycloakId to authentikId
+    private String authentikId;
     private String username;
 
     @Column(precision = 19, scale = 2)

backend/src/main/java/de/szut/casino/user/dto/KeycloakUserDto.java

@@ -10,8 +10,6 @@ import lombok.Setter;
 @AllArgsConstructor
 @NoArgsConstructor
 public class KeycloakUserDto {
-    // Renamed class but kept for backward compatibility
-    // This now contains Authentik user info
     private String sub;
     private String preferred_username;
 }