feat: 참조 사용자 응답에 이메일 필드 추가

Author: wngktjd13

backend/flowmeet-api/src/main/java/kr/flowmeet/api/chat/dto/response/ReferencedUserResponse.java

@@ -9,13 +9,16 @@ public record ReferencedUserResponse(
         Long userId,
         @Schema(description = "닉네임", example = "홍길동")
         String nickname,
+        @Schema(description = "이메일", example = "user@example.com")
+        String email,
         @Schema(description = "프로필 이미지 URL", example = "https://example.com/profile.png")
         String profileImageUrl
 ) {
     public static ReferencedUserResponse from(final ProjectMember member) {
         return new ReferencedUserResponse(
                 member.getUserId(),
                 member.getUser().getNickname(),
+                member.getUser().getEmail(),
                 member.getUser().getProfileImageUrl()
         );
     }