refacor:#4 DutyStatus Enum 클래스 생성 및 적용

src/main/java/com/example/kdtbe5_miniproject/duty/Duty.java

@@ -25,5 +25,5 @@ public class Duty {
 
     private LocalDate date;
     private String reason;
-    private String status;  //TODO enum 타입으로 변경 (대기, 승인, 반려)
+    private DutyStatus status;  //대기(0), 승인(1), 반려(2)
 }

src/main/java/com/example/kdtbe5_miniproject/duty/DutyResponse.java

@@ -12,7 +12,7 @@ public static class DutyDTO {
         private Long id;
         private LocalDate date;
         private String reason;
-        private String status;
+        private DutyStatus status;
 
         public DutyDTO(Duty duty) {
             this.id = duty.getId();

src/main/java/com/example/kdtbe5_miniproject/duty/DutyStatus.java

@@ -0,0 +1,14 @@
+package com.example.kdtbe5_miniproject.duty;
+
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+
+@Getter
+@RequiredArgsConstructor
+public enum DutyStatus {
+    대기(0),
+    승인(1),
+    반려(2);
+
+    private final int typeStatus;
+}