feat(chat): 채팅방 메시지 조회 API 구현

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

@@ -11,6 +11,7 @@ import org.springframework.web.bind.annotation.PathVariable
 import org.springframework.web.bind.annotation.PostMapping
 import org.springframework.web.bind.annotation.RequestBody
 import org.springframework.web.bind.annotation.RequestMapping
+import org.springframework.web.bind.annotation.RequestParam
 import org.springframework.web.bind.annotation.RestController
 
 @RestController
@@ -62,6 +63,24 @@ 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

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

@@ -24,6 +24,16 @@ data class ChatRoomListItemDto(
     val lastMessagePreview: String?
 )
 
+/**
+ * 채팅방 메시지 아이템 DTO (API 응답용)
+ */
+data class ChatMessageItemDto(
+    val messageId: Long,
+    val message: String,
+    val profileImageUrl: String,
+    val mine: Boolean
+)
+
 /**
  * 채팅방 목록 쿼리 DTO (레포지토리 투영용)
  */

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

@@ -8,4 +8,11 @@ import org.springframework.stereotype.Repository
 @Repository
 interface CharacterChatMessageRepository : JpaRepository<CharacterChatMessage, Long> {
     fun findTopByChatRoomAndIsActiveTrueOrderByCreatedAtDesc(chatRoom: CharacterChatRoom): CharacterChatMessage?
+
+    fun findTop20ByChatRoomAndIsActiveTrueOrderByIdDesc(chatRoom: CharacterChatRoom): List<CharacterChatMessage>
+
+    fun findTop20ByChatRoomAndIdLessThanAndIsActiveTrueOrderByIdDesc(
+        chatRoom: CharacterChatRoom,
+        id: Long
+    ): List<CharacterChatMessage>
 }

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

@@ -5,6 +5,7 @@ import kr.co.vividnext.sodalive.chat.character.service.ChatCharacterService
 import kr.co.vividnext.sodalive.chat.room.CharacterChatParticipant
 import kr.co.vividnext.sodalive.chat.room.CharacterChatRoom
 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.ChatRoomListItemDto
 import kr.co.vividnext.sodalive.chat.room.dto.ChatRoomListQueryDto
 import kr.co.vividnext.sodalive.chat.room.dto.CreateChatRoomResponse
@@ -324,4 +325,36 @@ class ChatRoomService(
         // 최종 실패 로그 (예외 미전파)
         log.error("[chat] 외부 세션 종료 최종 실패: sessionId={}, attempts={}", sessionId, maxAttempts)
     }
+
+    @Transactional(readOnly = true)
+    fun getChatMessages(member: Member, chatRoomId: Long, beforeMessageId: Long?): List<ChatMessageItemDto> {
+        val room = chatRoomRepository.findById(chatRoomId).orElseThrow {
+            SodaException("채팅방을 찾을 수 없습니다.")
+        }
+        val participant = participantRepository.findByChatRoomAndMemberAndIsActiveTrue(room, member)
+        if (participant == null) {
+            throw SodaException("잘못된 접근입니다")
+        }
+
+        val messages = if (beforeMessageId != null) {
+            messageRepository.findTop20ByChatRoomAndIdLessThanAndIsActiveTrueOrderByIdDesc(room, beforeMessageId)
+        } else {
+            messageRepository.findTop20ByChatRoomAndIsActiveTrueOrderByIdDesc(room)
+        }
+
+        return messages.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"}"
+            ChatMessageItemDto(
+                messageId = msg.id!!,
+                message = msg.message,
+                profileImageUrl = imageUrl,
+                mine = sender.member?.id == member.id
+            )
+        }
+    }
 }