Bug: `auth`在`mysql`下多次创建admin用户

### Describe the bug

`auth` 会upsert一个id=0的user admin
而mysql默认的auto_increment为1
导致在mysql环境下每次启动都会多一行admin

### Steps to reproduce

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### Expected behavior

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### Screenshots

_No response_

### Versions

- OS:
- Platform:
- Node version:
- Koishi version:

### Additional context

_No response_

Start by locating the auth code that upserts the id=0 admin user and trace how that record is persisted under MySQL. Reproduce the issue by restarting in a MySQL environment, then verify that startup no longer creates an additional admin row.

Written by the indexing model from the issue text.