设计 Post/Author/Tag 数据模型，补齐 CRUD、slug 校验、发布状态与权限控制。

[allovine]

文章发布后自动更新 sitemap、metadata、canonical。

Description

Describe your PR! If it fixes specific issue, mention it with "Fixes # (issue)".

Contributor Checklist

Make sure to do the following steps if they are applicable to your PR:

• Update e2e tests: If you changed the /template/app, then make sure to do any neccessary updates to /template/e2e-tests also.
• Update demo app: If you changed the /template/app, then make sure to do any neccessary updates to /opensaas-sh/app_diff also. Check /opensaas-sh/README.md for details.
• Update docs: If needed, update the /opensaas-sh/blog/src/content/docs.