fix(chat): 채팅방 메시지 전송 API 반환값 수정

- 기존: SendChatMessageResponse으로 메시지 리스트를 한 번 더 Wrapping해서 보냄

- 수정: 메시지 리스트 반환

src/main/kotlin/kr/co/vividnext/sodalive/chat/room/controller/ChatRoomController.kt

@@ -65,24 +65,6 @@ class ChatRoomController(
         }
     }
 
-    /**
-     * 채팅방 메시지 조회 API
-     * - 참여 여부 검증(미참여시 "잘못된 접근입니다")
-     * - messageId가 있으면 해당 ID 이전 20개, 없으면 최신 20개
-     */
-    @GetMapping("/{chatRoomId}/messages")
-    fun getChatMessages(
-        @AuthenticationPrincipal(expression = "#this == 'anonymousUser' ? null : member") member: Member?,
-        @PathVariable chatRoomId: Long,
-        @RequestParam(required = false) messageId: Long?
-    ) = run {
-        if (member == null) throw SodaException("로그인 정보를 확인해주세요.")
-        if (member.auth == null) throw SodaException("본인인증을 하셔야 합니다.")
-
-        val response = chatRoomService.getChatMessages(member, chatRoomId, messageId)
-        ApiResponse.ok(response)
-    }
-
     /**
      * 세션 상태 조회 API
      * - 채팅방 참여 여부 검증
@@ -100,6 +82,23 @@ class ChatRoomController(
         ApiResponse.ok(isActive)
     }
 
+    /**
+     * 채팅방 입장 API
+     * - 참여 여부 검증
+     * - 최신 20개 메시지를 createdAt 오름차순으로 반환
+     */
+    @GetMapping("/{chatRoomId}/enter")
+    fun enterChatRoom(
+        @AuthenticationPrincipal(expression = "#this == 'anonymousUser' ? null : member") member: Member?,
+        @PathVariable chatRoomId: Long
+    ) = run {
+        if (member == null) throw SodaException("로그인 정보를 확인해주세요.")
+        if (member.auth == null) throw SodaException("본인인증을 하셔야 합니다.")
+
+        val response = chatRoomService.enterChatRoom(member, chatRoomId)
+        ApiResponse.ok(response)
+    }
+
     /**
      * 채팅방 나가기 API
      * - URL에 chatRoomId 포함
@@ -119,6 +118,26 @@ class ChatRoomController(
         ApiResponse.ok(true)
     }
 
+    /**
+     * 채팅방 메시지 조회 API
+     * - 참여 여부 검증(미참여시 "잘못된 접근입니다")
+     * - cursor(메시지ID)보다 더 과거의 메시지에서 limit만큼 조회(경계 exclusive)
+     * - cursor 미지정 시 최신부터 limit만큼 기준으로 페이징
+     */
+    @GetMapping("/{chatRoomId}/messages")
+    fun getChatMessages(
+        @AuthenticationPrincipal(expression = "#this == 'anonymousUser' ? null : member") member: Member?,
+        @PathVariable chatRoomId: Long,
+        @RequestParam(defaultValue = "20") limit: Int,
+        @RequestParam(required = false) cursor: Long?
+    ) = run {
+        if (member == null) throw SodaException("로그인 정보를 확인해주세요.")
+        if (member.auth == null) throw SodaException("본인인증을 하셔야 합니다.")
+
+        val response = chatRoomService.getChatMessages(member, chatRoomId, cursor, limit)
+        ApiResponse.ok(response)
+    }
+
     /**
      * 채팅방 메시지 전송 API
      * - 참여 여부 검증(미참여시 "잘못된 접근입니다")

src/main/kotlin/kr/co/vividnext/sodalive/chat/room/dto/ChatRoomDto.kt

@@ -38,7 +38,17 @@ data class ChatMessageItemDto(
     val messageId: Long,
     val message: String,
     val profileImageUrl: String,
-    val mine: Boolean
+    val mine: Boolean,
+    val createdAt: Long
+)
+
+/**
+ * 채팅방 메시지 페이지 응답 DTO
+ */
+data class ChatMessagesPageResponse(
+    val messages: List<ChatMessageItemDto>,
+    val hasMore: Boolean,
+    val nextCursor: Long?
 )
 
 /**
@@ -116,13 +126,6 @@ data class SendChatMessageRequest(
     val message: String
 )
 
-/**
- * 채팅 메시지 전송 응답 DTO (캐릭터 메시지 리스트)
- */
-data class SendChatMessageResponse(
-    val characterMessages: List<ChatMessageItemDto>
-)
-
 /**
  * 외부 API 채팅 전송 응답 DTO
  */
@@ -151,3 +154,20 @@ data class ExternalCharacterMessage(
     @JsonProperty("timestamp") val timestamp: String,
     @JsonProperty("messageType") val messageType: String
 )
+
+/**
+ * 채팅방 입장 응답 DTO
+ */
+data class ChatRoomEnterCharacterDto(
+    val characterId: Long,
+    val name: String,
+    val profileImageUrl: String,
+    val characterType: String
+)
+
+data class ChatRoomEnterResponse(
+    val roomId: Long,
+    val character: ChatRoomEnterCharacterDto,
+    val messages: List<ChatMessageItemDto>,
+    val hasMoreMessages: Boolean
+)

src/main/kotlin/kr/co/vividnext/sodalive/chat/room/repository/ChatMessageRepository.kt

@@ -2,6 +2,7 @@ package kr.co.vividnext.sodalive.chat.room.repository
 
 import kr.co.vividnext.sodalive.chat.room.ChatMessage
 import kr.co.vividnext.sodalive.chat.room.ChatRoom
+import org.springframework.data.domain.Pageable
 import org.springframework.data.jpa.repository.JpaRepository
 import org.springframework.stereotype.Repository
 
@@ -9,10 +10,23 @@ import org.springframework.stereotype.Repository
 interface ChatMessageRepository : JpaRepository<ChatMessage, Long> {
     fun findTopByChatRoomAndIsActiveTrueOrderByCreatedAtDesc(chatRoom: ChatRoom): ChatMessage?
 
+    // 기존 20개 고정 메서드는 유지 (기존 호출 호환)
     fun findTop20ByChatRoomAndIsActiveTrueOrderByIdDesc(chatRoom: ChatRoom): List<ChatMessage>
 
     fun findTop20ByChatRoomAndIdLessThanAndIsActiveTrueOrderByIdDesc(
         chatRoom: ChatRoom,
         id: Long
     ): List<ChatMessage>
+
+    // 새로운 커서 기반 페이징용 메서드 (limit 가변)
+    fun findByChatRoomAndIsActiveTrueOrderByIdDesc(chatRoom: ChatRoom, pageable: Pageable): List<ChatMessage>
+
+    fun findByChatRoomAndIdLessThanAndIsActiveTrueOrderByIdDesc(
+        chatRoom: ChatRoom,
+        id: Long,
+        pageable: Pageable
+    ): List<ChatMessage>
+
+    // 더 이전 데이터 존재 여부 확인
+    fun existsByChatRoomAndIsActiveTrueAndIdLessThan(chatRoom: ChatRoom, id: Long): Boolean
 }

src/main/kotlin/kr/co/vividnext/sodalive/chat/room/service/ChatRoomService.kt

@@ -7,13 +7,15 @@ import kr.co.vividnext.sodalive.chat.room.ChatParticipant
 import kr.co.vividnext.sodalive.chat.room.ChatRoom
 import kr.co.vividnext.sodalive.chat.room.ParticipantType
 import kr.co.vividnext.sodalive.chat.room.dto.ChatMessageItemDto
+import kr.co.vividnext.sodalive.chat.room.dto.ChatMessagesPageResponse
+import kr.co.vividnext.sodalive.chat.room.dto.ChatRoomEnterCharacterDto
+import kr.co.vividnext.sodalive.chat.room.dto.ChatRoomEnterResponse
 import kr.co.vividnext.sodalive.chat.room.dto.ChatRoomListItemDto
 import kr.co.vividnext.sodalive.chat.room.dto.ChatRoomListQueryDto
 import kr.co.vividnext.sodalive.chat.room.dto.CreateChatRoomResponse
 import kr.co.vividnext.sodalive.chat.room.dto.ExternalChatSendResponse
 import kr.co.vividnext.sodalive.chat.room.dto.ExternalChatSessionCreateResponse
 import kr.co.vividnext.sodalive.chat.room.dto.ExternalChatSessionGetResponse
-import kr.co.vividnext.sodalive.chat.room.dto.SendChatMessageResponse
 import kr.co.vividnext.sodalive.chat.room.repository.ChatMessageRepository
 import kr.co.vividnext.sodalive.chat.room.repository.ChatParticipantRepository
 import kr.co.vividnext.sodalive.chat.room.repository.ChatRoomRepository
@@ -246,6 +248,70 @@ class ChatRoomService(
         return fetchSessionActive(room.sessionId)
     }
 
+    @Transactional(readOnly = true)
+    fun enterChatRoom(member: Member, chatRoomId: Long): ChatRoomEnterResponse {
+        val room = chatRoomRepository.findById(chatRoomId).orElseThrow {
+            SodaException("채팅방을 찾을 수 없습니다.")
+        }
+        // 참여 여부 검증
+        participantRepository.findByChatRoomAndMemberAndIsActiveTrue(room, member)
+            ?: throw SodaException("잘못된 접근입니다")
+
+        // 캐릭터 참여자 조회
+        val characterParticipant = participantRepository.findByChatRoomAndParticipantTypeAndIsActiveTrue(
+            room,
+            ParticipantType.CHARACTER
+        ) ?: throw SodaException("잘못된 접근입니다")
+
+        val character = characterParticipant.character
+            ?: throw SodaException("오류가 발생했습니다. 다시 시도해 주세요.")
+
+        val imageUrl = "$imageHost/${character.imagePath ?: "profile/default-profile.png"}"
+        val characterDto = ChatRoomEnterCharacterDto(
+            characterId = character.id!!,
+            name = character.name,
+            profileImageUrl = imageUrl,
+            characterType = character.characterType.name
+        )
+
+        // 메시지 최신 20개 조회 후 createdAt 오름차순으로 반환
+        val pageable = PageRequest.of(0, 20)
+        val fetched = messageRepository.findByChatRoomAndIsActiveTrueOrderByIdDesc(room, pageable)
+
+        val nextCursor: Long? = fetched.minByOrNull { it.id ?: Long.MAX_VALUE }?.id
+        val hasMore: Boolean = if (nextCursor != null) {
+            messageRepository.existsByChatRoomAndIsActiveTrueAndIdLessThan(room, nextCursor)
+        } else {
+            false
+        }
+
+        val messagesAsc = fetched.sortedBy { it.createdAt }
+        val items = messagesAsc.map { msg ->
+            val sender = msg.participant
+            val profilePath = when (sender.participantType) {
+                ParticipantType.USER -> sender.member?.profileImage
+                ParticipantType.CHARACTER -> sender.character?.imagePath
+            }
+            val senderImageUrl = "$imageHost/${profilePath ?: "profile/default-profile.png"}"
+            val createdAtMillis = msg.createdAt?.atZone(java.time.ZoneId.systemDefault())?.toInstant()?.toEpochMilli()
+                ?: 0L
+            ChatMessageItemDto(
+                messageId = msg.id!!,
+                message = msg.message,
+                profileImageUrl = senderImageUrl,
+                mine = sender.member?.id == member.id,
+                createdAt = createdAtMillis
+            )
+        }
+
+        return ChatRoomEnterResponse(
+            roomId = room.id!!,
+            character = characterDto,
+            messages = items,
+            hasMoreMessages = hasMore
+        )
+    }
+
     private fun fetchSessionActive(sessionId: String): Boolean {
         try {
             val factory = SimpleClientHttpRequestFactory()
@@ -358,39 +424,58 @@ class ChatRoomService(
     }
 
     @Transactional(readOnly = true)
-    fun getChatMessages(member: Member, chatRoomId: Long, beforeMessageId: Long?): List<ChatMessageItemDto> {
+    fun getChatMessages(member: Member, chatRoomId: Long, cursor: Long?, limit: Int = 20): ChatMessagesPageResponse {
         val room = chatRoomRepository.findById(chatRoomId).orElseThrow {
             SodaException("채팅방을 찾을 수 없습니다.")
         }
-        val participant = participantRepository.findByChatRoomAndMemberAndIsActiveTrue(room, member)
-        if (participant == null) {
-            throw SodaException("잘못된 접근입니다")
-        }
+        participantRepository.findByChatRoomAndMemberAndIsActiveTrue(room, member)
+            ?: throw SodaException("잘못된 접근입니다")
 
-        val messages = if (beforeMessageId != null) {
-            messageRepository.findTop20ByChatRoomAndIdLessThanAndIsActiveTrueOrderByIdDesc(room, beforeMessageId)
+        val pageable = PageRequest.of(0, limit)
+        val fetched = if (cursor != null) {
+            messageRepository.findByChatRoomAndIdLessThanAndIsActiveTrueOrderByIdDesc(room, cursor, pageable)
         } else {
-            messageRepository.findTop20ByChatRoomAndIsActiveTrueOrderByIdDesc(room)
+            messageRepository.findByChatRoomAndIsActiveTrueOrderByIdDesc(room, pageable)
         }
 
-        return messages.map { msg ->
+        // 가장 오래된 메시지 ID (nextCursor) 및 hasMore 계산
+        val nextCursor: Long? = fetched.minByOrNull { it.id ?: Long.MAX_VALUE }?.id
+        val hasMore: Boolean = if (nextCursor != null) {
+            messageRepository.existsByChatRoomAndIsActiveTrueAndIdLessThan(room, nextCursor)
+        } else {
+            false
+        }
+
+        // createdAt 오름차순으로 정렬하여 반환
+        val messagesAsc = fetched.sortedBy { it.createdAt }
+
+        val items = messagesAsc.map { msg ->
             val sender = msg.participant
             val profilePath = when (sender.participantType) {
                 ParticipantType.USER -> sender.member?.profileImage
                 ParticipantType.CHARACTER -> sender.character?.imagePath
             }
             val imageUrl = "$imageHost/${profilePath ?: "profile/default-profile.png"}"
+            val createdAtMillis = msg.createdAt?.atZone(java.time.ZoneId.systemDefault())?.toInstant()?.toEpochMilli()
+                ?: 0L
             ChatMessageItemDto(
                 messageId = msg.id!!,
                 message = msg.message,
                 profileImageUrl = imageUrl,
-                mine = sender.member?.id == member.id
+                mine = sender.member?.id == member.id,
+                createdAt = createdAtMillis
             )
         }
+
+        return ChatMessagesPageResponse(
+            messages = items,
+            hasMore = hasMore,
+            nextCursor = nextCursor
+        )
     }
 
     @Transactional
-    fun sendMessage(member: Member, chatRoomId: Long, message: String): SendChatMessageResponse {
+    fun sendMessage(member: Member, chatRoomId: Long, message: String): List<ChatMessageItemDto> {
         // 1) 방 존재 확인
         val room = chatRoomRepository.findById(chatRoomId).orElseThrow {
             SodaException("채팅방을 찾을 수 없습니다.")
@@ -441,10 +526,13 @@ class ChatRoomService(
             messageId = savedCharacterMsg.id!!,
             message = savedCharacterMsg.message,
             profileImageUrl = imageUrl,
-            mine = false
+            mine = false,
+            createdAt = savedCharacterMsg.createdAt?.atZone(java.time.ZoneId.systemDefault())?.toInstant()
+                ?.toEpochMilli()
+                ?: 0L
         )
 
-        return SendChatMessageResponse(characterMessages = listOf(dto))
+        return listOf(dto)
     }
 
     private fun callExternalApiForChatSendWithRetry(