Improve log observability of backend.

[zhuyuanmao]

While we were developing the backend of the Cello project, we didn't put much effort into structuring log information, which would be a painful technical debit in terms of maintainability and testability. We want to refine our log events for every error handling. Ultimately, we will achieve good observabilities and will be beneficial for long-term development. There is an example of PR we did for this task. #465

[syedmustafan]

The logs should be maintained.

Should create a new model for the logs. All the logs should be maintained in the table.

[zhuyuanmao]

The logs should be maintained.

Should create a new model for the logs. All the logs should be maintained in the table.

good point, but we don't have a plan to maintain the logs at this moment.