feat: 마이바(MyBar) 칵테일 킵(Keep) API 추가

- `POST /me/bar/{cocktailId}/keep` 엔드포인트 추가
- URL `@PathVariable`로 칵테일 ID를 받고, `@AuthenticationPrincipal`로 사용자 ID를 획득
- `MyBarService`의 `keep` 메서드를 호출하여 킵 기능 처리
- 성공 시 HTTP 201 상태 코드와 "kept" 메시지 반환

Author: MEOHIN

src/main/java/com/back/domain/mybar/controller/MyBarController.java

@@ -8,10 +8,7 @@
 import lombok.RequiredArgsConstructor;
 import org.springframework.security.core.annotation.AuthenticationPrincipal;
 import org.springframework.validation.annotation.Validated;
-import org.springframework.web.bind.annotation.GetMapping;
-import org.springframework.web.bind.annotation.RequestMapping;
-import org.springframework.web.bind.annotation.RequestParam;
-import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.bind.annotation.*;
 
 @RestController
 @RequestMapping("/me/bar")
@@ -30,4 +27,14 @@ public RsData<MyBarListResponseDto> getMyBarList(
         MyBarListResponseDto body = myBarService.getMyBar(userId, page, pageSize);
         return RsData.successOf(body);  // code=200, message="success"
     }
+
+    /** 킵 추가(생성/복원/재킵) */
+    @PostMapping("/{cocktailId}/keep")
+    public RsData<Void> keep(
+            @AuthenticationPrincipal(expression = "id") Long userId,
+            @PathVariable Long cocktailId
+    ) {
+        myBarService.keep(userId, cocktailId);
+        return RsData.of(201, "kept"); // Aspect가 HTTP 201로 설정
+    }
 }