feat: add key syncing and rotation

[oshinop]

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[AsakuraMizu]

@codex

[chatgpt-codex-connector]

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[AsakuraMizu]

感谢补充轮换和失败回滚能力。这个 PR 的主要问题是改变了现有契约：用户从“必须有且仅有一个 API Key”变成了允许无 Key，并为此引入了 nullable 字段、创建/删除、版本号、CAS 和同步逻辑。更严重的是，current_user() 每次都会读取上游全量 Key 并可能回写 SQLite，使普通认证请求依赖 CliRelay，同时让 Gate 和 CliRelay 各自保存一份需要同步的 Key，增加了不一致风险。建议保留失败回滚和显式轮换，但继续维持一用户一 Key，不支持创建/删除，也不要在读取用户时同步上游；这个 PR 需要按这个方向重新设计后再提交。