fix: 修复 notification_service 导入错误

添加 NotificationServiceAdapter 适配器类，兼容 API 层调用方式
导出 notification_service 单例实例

backend/app/services/notification_service.py

@@ -417,3 +417,186 @@ class NotificationService:
 def get_notification_service(db: AsyncSession) -> NotificationService:
     """获取通知服务实例"""
     return NotificationService(db)
+
+
+class NotificationServiceAdapter:
+    """
+    通知服务适配器
+    
+    提供静态方法接口，兼容 API 层的调用方式
+    每个方法接受 db 参数，内部创建 NotificationService 实例
+    """
+    
+    @staticmethod
+    async def get_user_notifications(
+        db: AsyncSession,
+        user_id: int,
+        skip: int = 0,
+        limit: int = 20,
+        is_read: Optional[bool] = None,
+        notification_type: Optional[str] = None,
+    ) -> tuple[List[Notification], int, int]:
+        """获取用户通知列表"""
+        from sqlalchemy import func
+        
+        # 构建查询条件
+        conditions = [Notification.user_id == user_id]
+        if is_read is not None:
+            conditions.append(Notification.is_read == is_read)
+        if notification_type:
+            conditions.append(Notification.type == notification_type)
+        
+        # 查询总数
+        total_stmt = select(func.count(Notification.id)).where(and_(*conditions))
+        total = await db.scalar(total_stmt) or 0
+        
+        # 查询未读数
+        unread_stmt = select(func.count(Notification.id)).where(
+            and_(Notification.user_id == user_id, Notification.is_read == False)
+        )
+        unread_count = await db.scalar(unread_stmt) or 0
+        
+        # 查询列表
+        list_stmt = (
+            select(Notification)
+            .where(and_(*conditions))
+            .order_by(Notification.created_at.desc())
+            .offset(skip)
+            .limit(limit)
+        )
+        result = await db.execute(list_stmt)
+        notifications = list(result.scalars().all())
+        
+        return notifications, total, unread_count
+    
+    @staticmethod
+    async def get_unread_count(
+        db: AsyncSession,
+        user_id: int,
+    ) -> tuple[int, int]:
+        """获取未读通知数量"""
+        from sqlalchemy import func
+        
+        # 未读数
+        unread_stmt = select(func.count(Notification.id)).where(
+            and_(Notification.user_id == user_id, Notification.is_read == False)
+        )
+        unread_count = await db.scalar(unread_stmt) or 0
+        
+        # 总数
+        total_stmt = select(func.count(Notification.id)).where(
+            Notification.user_id == user_id
+        )
+        total = await db.scalar(total_stmt) or 0
+        
+        return unread_count, total
+    
+    @staticmethod
+    async def mark_as_read(
+        db: AsyncSession,
+        user_id: int,
+        notification_ids: Optional[List[int]] = None,
+    ) -> int:
+        """标记通知为已读"""
+        from sqlalchemy import update
+        
+        if notification_ids:
+            # 标记指定通知
+            stmt = (
+                update(Notification)
+                .where(
+                    and_(
+                        Notification.user_id == user_id,
+                        Notification.id.in_(notification_ids),
+                        Notification.is_read == False
+                    )
+                )
+                .values(is_read=True, updated_at=datetime.now())
+            )
+        else:
+            # 标记所有未读
+            stmt = (
+                update(Notification)
+                .where(
+                    and_(
+                        Notification.user_id == user_id,
+                        Notification.is_read == False
+                    )
+                )
+                .values(is_read=True, updated_at=datetime.now())
+            )
+        
+        result = await db.execute(stmt)
+        await db.commit()
+        return result.rowcount
+    
+    @staticmethod
+    async def delete_notification(
+        db: AsyncSession,
+        user_id: int,
+        notification_id: int,
+    ) -> bool:
+        """删除通知"""
+        from sqlalchemy import delete
+        
+        stmt = delete(Notification).where(
+            and_(
+                Notification.id == notification_id,
+                Notification.user_id == user_id
+            )
+        )
+        result = await db.execute(stmt)
+        await db.commit()
+        return result.rowcount > 0
+    
+    @staticmethod
+    async def create_notification(
+        db: AsyncSession,
+        notification_in: Any,
+    ) -> Notification:
+        """创建单条通知"""
+        notification = Notification(
+            user_id=notification_in.user_id,
+            title=notification_in.title,
+            content=notification_in.content,
+            type=getattr(notification_in, 'type', 'system'),
+            related_id=getattr(notification_in, 'related_id', None),
+            related_type=getattr(notification_in, 'related_type', None),
+            sender_id=getattr(notification_in, 'sender_id', None),
+            is_read=False,
+        )
+        db.add(notification)
+        await db.commit()
+        await db.refresh(notification)
+        return notification
+    
+    @staticmethod
+    async def batch_create_notifications(
+        db: AsyncSession,
+        batch_in: Any,
+    ) -> List[Notification]:
+        """批量创建通知"""
+        notifications = []
+        for user_id in batch_in.user_ids:
+            notification = Notification(
+                user_id=user_id,
+                title=batch_in.title,
+                content=batch_in.content,
+                type=getattr(batch_in, 'type', 'system'),
+                related_id=getattr(batch_in, 'related_id', None),
+                related_type=getattr(batch_in, 'related_type', None),
+                sender_id=getattr(batch_in, 'sender_id', None),
+                is_read=False,
+            )
+            db.add(notification)
+            notifications.append(notification)
+        
+        await db.commit()
+        for n in notifications:
+            await db.refresh(n)
+        
+        return notifications
+
+
+# 全局单例适配器
+notification_service = NotificationServiceAdapter()