feat: relocate remaining dtos

backend/src/main/java/com/openisle/dto/ConfigDto.java

@@ -0,0 +1,15 @@
+package com.openisle.dto;
+
+import com.openisle.model.PasswordStrength;
+import com.openisle.model.PublishMode;
+import com.openisle.model.RegisterMode;
+import lombok.Data;
+
+/** DTO for site configuration. */
+@Data
+public class ConfigDto {
+    private PublishMode publishMode;
+    private PasswordStrength passwordStrength;
+    private Integer aiFormatLimit;
+    private RegisterMode registerMode;
+}