refactor(party): 재참여 파티원 고려

Author: seopgyu

src/main/java/com/back/matchduo/domain/party/entity/PartyMember.java

@@ -62,4 +62,11 @@ public void leaveParty() {
         this.state = PartyMemberState.LEFT;
         this.leftAt = LocalDateTime.now();
     }
+
+    // 나갔던 유저가 다시 들어올 때 사용
+    public void rejoinParty() {
+        this.state = PartyMemberState.JOINED;
+        this.joinedAt = LocalDateTime.now();
+        this.leftAt = null;
+    }
 }