feat(conversations): add notification mute functionality for conversations

- Rename Muted field to NotificationMuted across model, DTOs and converters
- Add UpdateNotificationMuted repository method with upsert support
- Add SetConversationNotificationMuted service method
- Add PUT /:id/notification_muted route for setting mute status
- Improve S3 storage resilience by skipping bucket check on access denied

internal/repository/message_repo.go

@@ -28,6 +28,7 @@ type MessageRepository interface {
 	GetParticipant(conversationID string, userID string) (*model.ConversationParticipant, error)
 	UpdateLastReadSeq(conversationID string, userID string, lastReadSeq int64) error
 	UpdatePinned(conversationID string, userID string, isPinned bool) error
+	UpdateNotificationMuted(conversationID string, userID string, notificationMuted bool) error
 	GetUnreadCount(conversationID string, userID string) (int64, error)
 	UpdateConversationLastSeq(conversationID string, seq int64) error
 	GetNextSeq(conversationID string) (int64, error)
@@ -337,6 +338,36 @@ func (r *messageRepository) UpdatePinned(conversationID string, userID string, i
 	return nil
 }
 
+// UpdateNotificationMuted 更新会话免打扰状态（用户维度）
+func (r *messageRepository) UpdateNotificationMuted(conversationID string, userID string, notificationMuted bool) error {
+	result := r.db.Model(&model.ConversationParticipant{}).
+		Where("conversation_id = ? AND user_id = ?", conversationID, userID).
+		Update("muted", notificationMuted)
+
+	if result.Error != nil {
+		return result.Error
+	}
+
+	if result.RowsAffected == 0 {
+		return r.db.Clauses(clause.OnConflict{
+			Columns: []clause.Column{
+				{Name: "conversation_id"},
+				{Name: "user_id"},
+			},
+			DoUpdates: clause.Assignments(map[string]any{
+				"muted":       notificationMuted,
+				"updated_at":  gorm.Expr("CURRENT_TIMESTAMP"),
+			}),
+		}).Create(&model.ConversationParticipant{
+			ConversationID:   conversationID,
+			UserID:           userID,
+			NotificationMuted: notificationMuted,
+		}).Error
+	}
+
+	return nil
+}
+
 // GetUnreadCount 获取未读消息数
 // userID 参数为 string 类型（UUID格式），与JWT中user_id保持一致
 func (r *messageRepository) GetUnreadCount(conversationID string, userID string) (int64, error) {