feat(chat): 채팅방 입장 API와 메시지 페이징 초기 로드 구현

- GET /api/chat/room/{chatRoomId}/enter 엔드포인트 추가
- 참여 검증 후 roomId, character(아이디/이름/프로필/타입) 제공
- 최신 20개 메시지 조회(내림차순 조회 후 createdAt 오름차순으로 정렬)
- hasMoreMessages 플래그 계산(가장 오래된 메시지 이전 존재 여부 판단)

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

@@ -82,6 +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

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

@@ -161,3 +161,20 @@ 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/service/ChatRoomService.kt

@@ -8,6 +8,8 @@ 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
@@ -247,6 +249,70 @@ 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()