fix(seed): preserve CAS cloud metadata

[wumingzhinu]

Summary / 摘要

• 更新 CAS codec 回归测试以覆盖当前 CasPayload 字段：cloud、slice_md5s、slice_size。

• 修复单文件 CAS 解码时丢失顶层 cloud 的问题，确保自定义云盘标识在解析和再次编码后保持不变。

• 增加 cloud=115 的往返测试。

• This PR has breaking changes.
  / 此 PR 包含破坏性变更。

• This PR changes public API, config, storage format, or migration behavior.
  / 此 PR 修改了公开 API、配置、存储格式或迁移行为。

• This PR requires corresponding changes in related repositories.
  / 此 PR 需要关联仓库同步修改。

Related repository PRs / 关联仓库 PR:

• OpenList: none
• OpenList-Docs: none

Testing / 测试

• node node_modules/tsx/dist/cli.mjs --test src/backend/server/seed.test.ts — 6/6 passed
• git diff --check
• Full typecheck — 2 pre-existing errors in src/backend/internal/model/db_cipher.test.ts:187-188
• Prettier check — the two touched files were already not Prettier-clean at baseline; no unrelated whole-file reformat was included

Review Notes

• Existing non-default multi-file slice-size behavior remains a separate follow-up; this PR only fixes single-file cloud metadata preservation.

Checklist / 检查清单

• I have read CONTRIBUTING.
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• I confirm this contribution follows the repository license, contribution policy, and code of conduct.
  / 我确认本次贡献符合仓库许可证、贡献规范和行为准则。
• I have formatted the changed code with Prettier where applicable.
  / 变更文件基线本身未通过 Prettier，本次未进行无关的全文件重排。
• I have requested review from relevant maintainers or code owners where applicable.
  / 我已在适用情况下请求相关维护者审查。

AI Disclosure / AI 使用声明

• This PR includes AI-assisted content.
  / 此 PR 包含 AI 辅助内容。

Tools used / 使用工具:

• Other: OpenCode

Usage scope / 使用范围:

• Code generation / 代码生成

• Tests / 测试

• Review assistance / 审查辅助

• I have reviewed and validated all AI-assisted content included in this PR.
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• I have ensured that all AI-assisted commits include Co-Authored-By attribution.
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• I can reproduce all AI-assisted content included in this PR without any AI tools.
  / 我可以在没有 AI 工具的情况下重现此 PR 的内容。