fix(backend): deleted remote users could be shown in `users/show` endpoint

[zyoshoka]

What

users/show エンドポイントに削除したリモートユーザーを指定しても取得できてしまう問題を修正

Why

Closes #14728

Additional info (optional)

他のエンドポイントでも塞ぐべきでしょうがそもそも isSuspended についてもそうなっていない、というケースが多いようなのでひとまず users/show エンドポイントのみの対応としています

Checklist

• [x] Read the contribution guide
• [x] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [x] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests

[github-actions[bot]]

このPRによるapi.jsonの差分 差分はありません。 Get diff files from Workflow Page

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 38.54%. Comparing base (12bc671) to head (08a6260).

Files with missing lines	Patch %	Lines
...ges/backend/src/server/api/endpoints/users/show.ts	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14752 +/- ## =========================================== - Coverage 39.66% 38.54% -1.13% =========================================== Files 1553 1553 Lines 195050 195051 +1 Branches 2574 3320 +746 =========================================== - Hits 77363 75175 -2188 - Misses 117119 119269 +2150 - Partials 568 607 +39 ```

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