Each section should have length validation

Each section should receive a `maxLength` property from the API which is then used on the UI to impose length restrictions. This is to ensure that the final resume generated complies well with our standard templates and remains optimum for ATS scans.

This perhaps needs breaking down a bit. As we will refactor the backend first, and then implement this in the frontend.

該 issue 先提到 backend 重構，接著是 frontend 實作。尋找定義 resume sections 的 API models 或 schemas，以新增 maxLength 屬性。然後，找到用於 resume sections 的 frontend form components，以套用長度驗證。檢查現有評論，看看是否有任何設計討論或部分工作。

由索引模型根據 Issue 內容生成。