feat: add channel support

backend/src/main/java/com/openisle/service/ChannelService.java

@@ -0,0 +1,76 @@
+package com.openisle.service;
+
+import com.openisle.dto.ChannelDto;
+import com.openisle.model.MessageConversation;
+import com.openisle.model.MessageParticipant;
+import com.openisle.model.User;
+import com.openisle.repository.MessageConversationRepository;
+import com.openisle.repository.MessageParticipantRepository;
+import com.openisle.repository.MessageRepository;
+import com.openisle.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.time.LocalDateTime;
+import java.util.List;
+import java.util.stream.Collectors;
+
+@Service
+@RequiredArgsConstructor
+public class ChannelService {
+    private final MessageConversationRepository conversationRepository;
+    private final MessageParticipantRepository participantRepository;
+    private final MessageRepository messageRepository;
+    private final UserRepository userRepository;
+
+    @Transactional(readOnly = true)
+    public List<ChannelDto> listChannels(Long userId) {
+        List<MessageConversation> channels = conversationRepository.findByChannelTrue();
+        return channels.stream().map(c -> toDto(c, userId)).collect(Collectors.toList());
+    }
+
+    @Transactional
+    public ChannelDto joinChannel(Long channelId, Long userId) {
+        MessageConversation channel = conversationRepository.findById(channelId)
+                .orElseThrow(() -> new IllegalArgumentException("Channel not found"));
+        User user = userRepository.findById(userId)
+                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+        participantRepository.findByConversationIdAndUserId(channelId, userId)
+                .orElseGet(() -> {
+                    MessageParticipant p = new MessageParticipant();
+                    p.setConversation(channel);
+                    p.setUser(user);
+                    MessageParticipant saved = participantRepository.save(p);
+                    channel.getParticipants().add(saved);
+                    return saved;
+                });
+        return toDto(channel, userId);
+    }
+
+    private ChannelDto toDto(MessageConversation channel, Long userId) {
+        ChannelDto dto = new ChannelDto();
+        dto.setId(channel.getId());
+        dto.setName(channel.getName());
+        dto.setDescription(channel.getDescription());
+        dto.setAvatar(channel.getAvatar());
+        dto.setMemberCount(channel.getParticipants().size());
+        boolean joined = channel.getParticipants().stream()
+                .anyMatch(p -> p.getUser().getId().equals(userId));
+        dto.setJoined(joined);
+        if (joined) {
+            MessageParticipant participant = channel.getParticipants().stream()
+                    .filter(p -> p.getUser().getId().equals(userId))
+                    .findFirst().orElse(null);
+            LocalDateTime lastRead = participant.getLastReadAt() == null
+                    ? LocalDateTime.of(1970, 1, 1, 0, 0)
+                    : participant.getLastReadAt();
+            long unread = messageRepository
+                    .countByConversationIdAndCreatedAtAfterAndSenderIdNot(channel.getId(), lastRead, userId);
+            dto.setUnreadCount(unread);
+        } else {
+            dto.setUnreadCount(0);
+        }
+        return dto;
+    }
+}