fix(db): 使用 lambda 表达式确保 updated_at 字段正确更新

[SXP-Simon]

Modifications / 改动点

updated_at 字段原先在 sa_column_kwargs["onupdate"] 中直接使用了 datetime.now()，导致时间戳仅在模块导入时被计算一次，之后不再变化，结果所有记录的更新时间都被设成了程序启动时间。

本次修改将时间戳生成逻辑封装进 lambda 表达式，使 SQLAlchemy 在每次更新记录时才惰性求值，从而保证时间戳实时更新。

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。/ If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
• 👀 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。/ My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
• 🤓 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到了 requirements.txt 和 pyproject.toml 文件相应位置。/ I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
• 😮 我的更改没有引入恶意代码。/ My changes do not introduce malicious code.

Summary by Sourcery

错误修复：

• 修复多个 SQLModel 实体中的 updated_at 字段，使其时间戳在更新时评估，而不是仅在导入时评估一次。

Original summary in English

Summary by Sourcery

Bug Fixes:

• Fix updated_at fields across multiple SQLModel entities so their timestamps are evaluated at update time instead of only once at import.

[sourcery-ai]

Hey - 我在这里给出一些总体反馈：

• 由于 created_at/updated_at 在多个模型中有完全相同的定义，可以考虑把它们提取成一个可复用的 mixin 或辅助函数，以避免重复并保持时间戳行为一致。
• 如果你希望以数据库时间作为唯一可信来源，可以考虑使用 func.now()（或等效的服务端默认值/onupdate）来代替 datetime.now(timezone.utc)，这样更新时间就由数据库而不是应用自身的时钟来驱动。

给 AI Agent 的提示

Please address the comments from this code review:

## Overall Comments
- Since `created_at`/`updated_at` share identical definitions across multiple models, consider extracting them into a reusable mixin or helper function to avoid duplication and keep timestamp behavior consistent.
- If you want the database time to be the single source of truth, you might consider using `func.now()` (or an equivalent server-side default/onupdate) instead of `datetime.now(timezone.utc)` so updates are driven by the DB rather than application clock.

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Original comment in English

Hey - I've left some high level feedback:

• Since created_at/updated_at share identical definitions across multiple models, consider extracting them into a reusable mixin or helper function to avoid duplication and keep timestamp behavior consistent.
• If you want the database time to be the single source of truth, you might consider using func.now() (or an equivalent server-side default/onupdate) instead of datetime.now(timezone.utc) so updates are driven by the DB rather than application clock.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Since `created_at`/`updated_at` share identical definitions across multiple models, consider extracting them into a reusable mixin or helper function to avoid duplication and keep timestamp behavior consistent.
- If you want the database time to be the single source of truth, you might consider using `func.now()` (or an equivalent server-side default/onupdate) instead of `datetime.now(timezone.utc)` so updates are driven by the DB rather than application clock.

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