fix: 重命名通知渠道模型避免与 alert 模块冲突

- NotificationChannel -> TaskNotifyChannel
- platform_notification_channels -> platform_task_notify_channels

backend/app/models/__init__.py

@@ -9,7 +9,7 @@ from .logs import PlatformLog
 from .alert import AlertRule, AlertRecord, NotificationChannel
 from .pricing import ModelPricing, TenantBilling
 from .scheduled_task import ScheduledTask, TaskLog, ScriptVar, Secret
-from .notification_channel import NotificationChannel
+from .notification_channel import TaskNotifyChannel
 
 __all__ = [
     "Tenant",
@@ -31,5 +31,5 @@ __all__ = [
     "TaskLog",
     "ScriptVar",
     "Secret",
-    "NotificationChannel"
+    "TaskNotifyChannel"
 ]