🐛 [Fix]: 팔로우 이메일 로직 수정

src/main/java/dongguk/osori/domain/follow/FollowController.java

@@ -65,18 +65,18 @@ public ResponseEntity<MyFollowResponse> getMyFollowerList(@SessionAttribute(name
             @ApiResponse(responseCode = "404", description = "해당 이메일의 사용자를 찾을 수 없음")
     })
     @PostMapping()
-    public ResponseEntity<Void> followUserByEmail(@SessionAttribute(name = "userId", required = false) Long userId, @RequestBody FollowRequestDto followRequestDto) {
+    public ResponseEntity<Void> followUserByEmail(@SessionAttribute(name = "userId", required = false) Long userId,
+                                                  @RequestBody FollowRequestDto followRequestDto) {
         if (userId == null) {
             return ResponseEntity.status(401).build();
         }
 
-        User userToFollow = userService.findUserByEmail(followRequestDto.getEmail());
-        if (userToFollow == null) {
-            return ResponseEntity.status(404).build();
+        try {
+            followService.followUserByEmail(userId, followRequestDto.getEmail());
+            return ResponseEntity.ok().build();
+        } catch (IllegalArgumentException e) {
+            return ResponseEntity.status(404).body(null);
         }
-
-        followService.followUser(userId, userToFollow.getUserId());
-        return ResponseEntity.ok().build();
     }
 
     @Operation(summary = "언팔로우", description = "사용자를 언팔로우합니다.")

src/main/java/dongguk/osori/domain/follow/service/FollowService.java

@@ -80,6 +80,23 @@ public void followUser(Long userId, Long followingUserId) {
         followRepository.save(follow);
     }
 
+    @Transactional
+    public void followUserByEmail(Long userId, String email) {
+        User follower = userRepository.findById(userId)
+                .orElseThrow(() -> new IllegalArgumentException("Logged-in user not found with id: " + userId));
+
+        User following = userRepository.findByEmail(email)
+                .orElseThrow(() -> new IllegalArgumentException("User not found with email: " + email));
+
+        boolean alreadyFollowing = followRepository.findByFollowerAndFollowing(follower, following).isPresent();
+        if (alreadyFollowing) {
+            throw new IllegalArgumentException("You are already following this user.");
+        }
+
+        followRepository.save(new Follow(follower, following));
+    }
+
+
     // 언팔로우
     @Transactional
     public void unfollowUser(Long userId, Long followingId) {