Add a new slash command `/user_stat` to fetch and display user statistics.
* **bot.py**
- Add a new slash command `/user_stat` to fetch and display the current input token, output token, and model for the user.
- Retrieve the user's history to calculate the input and output tokens.
- Fetch the model from the database.
- Update the `help_command` to include the new `/user_stat` command.
* **README.md**
- Add documentation for the new `/user_stat` command.
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