feat: проверка существования канала при привязке

src/domain/error.py

@@ -31,6 +31,12 @@ def ChannelNotFound(channel_id: uuid.UUID | None = None) -> HTTPException:
     return HTTPException(status.HTTP_404_NOT_FOUND, f'Channel {channel_id} not found')
 
 
+def TelegramChannelNotFound(username: str) -> HTTPException:
+    return HTTPException(
+        status.HTTP_404_NOT_FOUND, f'Telegram channel @{username} not found or is not a public channel'
+    )
+
+
 def ProjectNotFound(project_id: uuid.UUID | None = None) -> HTTPException:
     if project_id is None:
         return HTTPException(status.HTTP_404_NOT_FOUND, 'Project not found')